A potion of blindness makes you blind. If you drink one, it will make you blind for 250-450 (more) turns (if blessed, subtract 125; if cursed, add 125). Dipping a unicorn horn into this, a potion of confusion, or a potion of hallucination will produce an uncursed potion of water.
This potion can be used against monsters to blind them, or as a substitute for a blindfold if you have telepathy or are fighting a creature with a gaze attack.