I believe that the only bosses that have gained extra immunities are the orb guardians and Andor, and they are definitely not all immune to blindness. My recollection was that the idea would be that each boss would pick up a few (different) immunities, but not all of them. Other bosses such as cat lord, MaLaKaI, ancient blue dragon, minotaur emperor, are unchanged AFAIK.

Phial returning would be ridiculously overpowered, IMHO.