Speculah: Lee Sin is only pretending to be blind.

http://weknowmemes.com/wp-content/uploads/2013/09/day-1354-they-still-think-im-blind.jpg

Obvious developmental explanation aside. I assumed it was the Summoner who is acting as Lee's eyes, but since Riot commited genocide on Summoners, that's out of the question now. We now have Rek'sai and proof of exactly how a blind champion who can see function in-game when she is burrowed. But because Lee Sin doesn't rely on shockwaves to see like Rek does, this means he isn't blind, and that means he was pretending to be blind all ALONG!

And that means...that means there IS a reason why Teemo's Blinding Dart works on Lee Sin!!

TL;DR - Riot released to justify why can blind