Blindness is a status ailment, often permanent, which renders the player unable to see. The game screen literally becomes black, with only text descriptions of tactile sensations (such as "you've been hit by something!") and sounds ("you here a monster of some type near you"). The identify command doesn't work at all, while the commands that prompt you to select an item from your inventory (such as drop or ingest) no longer prompt you, instead performing the action on a random item.
Blindness can be cured by specifically asking the Gnome Crimelord to reverse it-- assuming you can find him-- or by consuming the Mama Mia Pasta Sauce. Since, however, the player will ingest items at random while blind, you might end up ingesting something you didn't want to, such as hemlock. This is one reason reason why most players are weary of carrying poisonous comestibles.
Blindness renders the player immune to the gorgon's stare and gives you an automatic 100 point karma rating. Most players agree however that these benefits are nowhere near the hassle blindness entails.
If you attempt to mate with thin air (or, um, "self-mate"), you will increase your chances of going blind naturally by 8%. Blindness can be hereditary, or can result from a casting of Magoo. A temporary blindness can result from Bedazzle, usually lasting 10 to 20 turns.