I always thought of 'shrugging off a bolt' as getting hit, but having no effect. When a PC dodges a bolt, it's a different message.

Well, so can a blinded PC.

Did you try this with other shopkeepers? The casino one is deliberately very tough and probably used to people trying to steal from him :p

I actually feel exactly the opposite. In fact, when I first started playing ADOM, I thought 'shrugging off' meant they were immune to it. It creates weird situations like fire elementals shrugging off frost bolts. How can they be weak to it sometimes, and then completely ignore it other times?

A message saying they avoided it would probably be more accurate.

This would make blinding ridiculously overpowered, IMHO. The current behaviour is probably intentional.

It's (almost) possible to beat the game while blind, so the disadvantage isn't that severe...

IMHO blinding as it is, is ridiculously underpowered.
NPC behavior doesn't make sense with the current setup, plain and simple. Blinding is blinding - with all the penalties it brings.
Either change the effects of successful hit with a thrown potion of blindness to much more severe or make the shopkeeper dodge the potion every time.

There is no middle ground when discussing sight and blindness. It's a zero - one scenario.
Even if the current behavior is intentional, I still consider it a serious oversight/bug and something that should be definitelly overhauled/rethought or at least addressed by TB to clear all doubts.
It's his game and his decission even if I don't have to agree with it.
All I ask for is that it makes logical sense - if it would be too powerful, make it extremely rare or make monsters dodge it much more frequently, respecting player skills with thrown items.
But when it hits and blinds, make it so that the target truly suffers all the consequences and is unable to hit the PC, much less crit him/her, hit 5 times in a row, follow him/her around, shrug bolts and dodge arrows.

The casino shopkeeper is not an exception here, there are many powerful monsters that share the same behavior when blinded, like wyrms, ancient *foo*, etc, though not all.
As for the definition of "shrug off" - I always understood it as the ability to redirect the bolt just before it hits so that it misses the target. Sometimes it can be successful even against bolts that a monster in question is supposed to be vulnerable to. It's not because of some random temporary immunity but because of the dodge-like ability to avoid being hit.

Overall I like many of the recent changes the game underwent in several aspects but I can't stop getting a feeling that the various limitations/intentional behaviors/additional abilities of monsters, etc. only serve to make it less and less consequential and more tedious.
All that due to so many elements not working the way a player with common sense would expect. If this is the driving idea for the future versions, then I'm going back to to 1.1.1 or at least 1.2.0pre5.

You know that motion you make when there's a mosquito flying around you and you want to swat it? I thought of that when I first saw the message about monsters "shrugging off" bolts. Still do sometimes.

Not necessarily. Many creatures are far less dependent on vision than humans are. Why should it necessarily be the case that a dragon, say, is significantly hampered by blindness? They may have other senses that are more important.

The behaviour of blind monsters is consistent with other games in the genre. Heck, if we wanted to do it your way, why can't monsters always instakill you when you're paralysed? Why do poisons only last minutes of game time instead of being debilitating for months? Why don't all wounds bleed? Why can the player go days without eating and never drink? Why can you go from average strength to giant strength in 10 days? Why is the player able to carry a semi truck worth of gear on his back and shoot a bow while wielding a two-handed weapon, berserk, in a suit of platemail armour? Rule of fun. Logical consistency takes a back seat to making the game playable and enjoyable.

I think about the monster shrugging its shoulders and the bolt going just above a shoulder.

Excuse the wall of text, you might want to jump straight to the last paragraph of this post if you don't want to waddle through ramblings of a disgruntled player:

I can agree that perhaps dragons can do without sight, although it's still a stretch in my book.
A bulette or an ant can go without it for sure. A dark elf maybe or an orc can reasonably well function without sight.
However the main point of my original post was related to shopkeepers, and mostly human/humanoidal ones.
Without sight, the sense that provides over 90% of general senses' input, such a being even if previously trained to function without it, will have a hard time performing any combat activities with any reasonable efficiency or skill, and that cannot be argued.

Logical consistency can go side by side with fun, if done right.
In fact, many would argue that logical consistency improves the fun factor.
Of course Ancardia is a magical place and lots of events taking place here are supernatural by nature and defy attempts to analyze and quantify, as well as go beyond the boundaries of widely understood physics.
There's a lot of things in ADOM that are inconsistent logically, I agree. Many of them could be tweaked to at least keep the appearance of consistency.
Most of those I got used to by playing the game for a long time, but since a lot of things is being changed, I was hoping that perhaps the direction of changes was not to impose more limits but to make the game be more reasonable and the mechanics make more sense. The problem I see here is the line between making the game difficult/removing scumming methods and keeping it consistent.
I believe this border has been crossed.
I see that more and more elements of the game mechanics that have been discovered by different people and used to their advantage, are being removed from the game because they apparently make it easier than intended/too easy overall.

This isn't rewarding players' ingenuity and thinking, this is punishing them for daring to extensively use tricky techniques to make life easier; techinques they spent endless hours of gameplay and countless dead PCs to discover in the first place.

Imagine you're some god and you created a world. Suddenly you find out that someone has figured out a way to create gold using lead, with a very simple and cheap method.
He has obtained a way of going through his life with ease and either does or doesn't want to share his discovery with fellow beings.
Do you decide to change the fundamental laws of the world and the atomic structure of some elements because your world is flawed?
Even if most or all of the other elements of that world still work properly?
Or do you chose to let it run its course and live with the consequences of your earlier decissions?

Anyway this goes somewhat off topic so I'll return to the original issue:
Rather than potion of blinding it could be called a potion of disability or potion of handicap to reflect its current function and effect it has on monsters.
Instead of blinding completely (which obviously it doesn't do at the moment, despite the name) it could blind partially, but also maybe slow down by 20-30 points or induce range/to-hit and/or to-damage penalties. Because as it is right now, it's a potion of blindiness only by name and by no means by function.

I agree with NPC blindness lowering DV and to-hit, but isn't that already present in the game? Apart from the fact that thrown potions always hitting a monster (100% chance) has always seemed a little weird to me, I don't have a problem with the way NPC blindness works at the moment. Also keep in mind that a handful of big bosses now resist certain kinds of disabling attacks.

As for shrugging bolts, I always considered it an effect of innate resistance to magic, not of dodging ability.

Potions of blindness miss occasionally at least in 1.2.0pre11.
For all I care, they can miss most of the time, or the hit/miss chance should depend on thrown weapons marks.
That is not the problem, the problem is the way they work when they hit, that is what I consider a bug. They don't work as advertised since they don't completely blind the target.
Perhaps they should be named potions of dim vision or potions of milky eyes to match the real effects they have on targets and there won't be any more issues.

As for shrugging being related to resistance, how can you explain what someone has already said: that fire elementals can occasionally shrug off frost bolts which they are supposed to be additionally vulnerable to?
Same goes for red/white dragons/wyrms occasionally shruging frost/fire bolts. Various monsters shrugging paralyzation bolts when otherwise they are susceptible to them.
I think shrugging means dodging in a way, hence the chance to avoid being hit no matter the nature of the magical attack.
Also, monsters (shopkeepers included) that shrug off bolts of one type at the same time can be badly injured by ball spells of the same type (ice/fire/acid/shock). This again points out that shrugging off bolts is unrelated to resistance.
Ancient blue dragons shrug off lightning bolts when they could just move on since they are naturally immune to them.
When they shrug the first one, the monster memory points out that they are immune to it, but when some other monster that is not resistant/immune to a particular element shrugs it off, similar entry does not appear in the monster description, hence there is no immunity.
The way I understand resistance is it blocks large part of the related element, but when a bolt is shrugged off, there is no damage dealt at all.
This brings us to another conclusion: shrugging off is neither immunity nor resistance related.

Indeed, "shrugged off" does seem like the NPC avoided the bolt rather than resisted it. Since
a/ there's no (other) message for an NPC explicitly dodging a bolt, even though the PC can do this;
b/ an NPC vulnerable to a bolt can shrug it completely, resulting in a strange situation where they either feel no pain at all or get very badly hurt;
c/ an NPC immune to an element can get the "shrugs off" message, which is different to the one that occurs when they actually resist it (something like "the <foo> resists the {searing flames, etc}").

What I first thought when I saw this issue was, what if a blind NPC can still sense a bolt coming, like the way the PC uses the Alertness skill to avoid bolts? Since bolts seem to make a noise when they are fired (at least when zapped from wands), it might be possible that the NPC might still hear it and guess its vector... But then, I noticed the point made in the OP that blind NPCs "stagger aimlessly" when blind, which seems to contradict this idea of using their senses to navigate.

So maybe then, for consistency, either this is a bug as Blasphemous says or blind NPCs "staggering aimlessly" shouldn't occur? After all, if an invisible PC attacks an NPC and flees, the NPC can still track them down despite the fact it can't see them...