Why isn't Teemo's blind like Graves'?
While the player's screen goes dark when in smoke from Graves, nothing of the sort happens to players blinded by Teemo's Q.

While the player's screen goes dark when in smoke from Graves, nothing of the sort happens to players blinded by Teemo's Q.

Bad wording.
Teemo's/Quinn's Blind makes auto-attacks miss.
Grave's/Nocturne's take away vision of the map.
Both are meant to serve different purposes.
Teemo's Blinding Dart pretty much makes the enemy's basic attacks useless.
Like when a champion is silenced and can't use abilities, it's making the enemy's basic attacks silenced.
While Grave's attack slows movement and disrupts vision.
Having a screen-wide display change would probably be too annoying. Your champion does have a blind indicator (black cloud over your champion's face for Teemo).
Blind =/= Vision Reduction. Blind is a form of CC that causing autoattacks to miss. Vision reduction effects cause a champ to lose allied vision and roughly half of their own vision radius. This is like asking why Annie's spells don't burn people like Brand's do. Because she's a different champion. She does different things.