League of Legends: How do blind effects work?

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.

For future reference, here's a copy of the lol wiki Blind article:

Blind

• The target will miss auto-attacks, incurring the message "Miss" above their attack target.
  • Blind only mitigates the physical damage portion of an attack, including physical damage sourced from a physical on-hit effect.
  • Other damage types and effects won't miss. See: On-hit effects.
  • Abilities that apply on-hit effects will also miss.
  • Blind is considered a form of damage mitigation. As such, it is applied after Thornmail's passive.
• Does not interrupt channeling.
• The duration is affected by crowd control reduction.
• The effect can be removed by all cleansing effects (except for Mikael's Crucible).
• Blind will not affect certain champion abilities.(e.g. Riven's passive Runic Blade)
• Champions with Blind: Heimerdinger, Teemo , Quinn and Valor, Shaco (non-champions only)