Currently the Sunglasses and Ankh Shield provide complete immunity to Blindness. However, certain mods have effects that apply higher levels of Blindess (Blindness II/Blindness III) as debuffs.
Ice & Fire applies it via the Blindfold to provide challenge to facing gorgons and cockatrice.
Better with Mods has it in the higher levels of Gloom as the player is assaulted by supernatural darkness.

It doesn't make any sense that wearing sunglasses would allow you to wear a blindfold and see normally. Could there be an option in the config that these 2 baubles only provide immunity to Blindness I?