Blind affects auto-attacks only. Instead of damage numbers, a yellow "MISS" will display instead.
Blind interacts oddly with ON-NEXT-HIT spells, like Garen's Decisive Strike and Xin Zhao's 3-talon strike. The Auto-attack damage is negated, but any spell effect still goes through.
This means, for instance, that Xin will knock your champion up on a 3rd hit, even when he's blind.
"Blind has no effect on Lee Sin the Blind Monk."
In an April Fool's video, Riot joked that Lee Sin was immune to the effects of blind, but this is not the case.