Darkness should work similarly to blindness, and in both cases, there should be three behavioural elements for those affected by it (as opposed to those that see in darkness or are immune to blindness):
1. A tendency to keep going in the direction they're going unless given reason to do otherwise (like being attacked, running into a wall, reacting to a sound, etc).
2. If attacked, will attempt to attack back in the direction that the attack came from. Similarly, if hearing a sound, will move towards or away from it, depending on the sound. If it's a wall that is blocking, choose a random new direction to move in.
3. If walking into the location of another creature without being attacked, the attack should be slower than normal. In this case, if the monster walks into the PC at the right time, it should merely bump into the PC first ("Something bumped into you"). If the PC gets to move before the monster is able to attack, the PC can move out of the way. They shouldn't be two separate steps, however.
The ability to have some sense of which direction an attack came from in darkness should also be available to the PC. Perhaps a dark "?" symbol could be placed for that turn in the location that an attack came from. It wouldn't be persistent, though, and would disappear on the next turn.