On a random thought process...

When the eyes lose their sight, it normally refers to both eyes. It might be possible that Kakashi didn't go blind because Obito's eye didn't, which could be attributed to the Hashirama DNA in his body which prevented his eyes from going blind by either rejuvenating them or lessening the effects significantly. This might also explain why Kakashi isn't blind.

Not possible when looked at from a logical perspective. But this is Naruto...