How do blind effects such as Teemo's blinding dart and Heimer's grenade work in League of Legends? For example, does it work against an enhanced auto-attack such as Nasus' siphoning strike?
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.
In an April Fool's video, Riot joked that Lee Sin was immune to the effects of blind, but this is not the case.
For future reference, here's a copy of the lol wiki Blind article: