Why isn't Tobi blind yet?
This answer contains lots of spoilers so I might as well make one block, edit if you will


   When Madara rescued Obito from the collapsed cave he used Hashirama
 Senju's living clone to heal Obito's crushed body. Not only did this
 heal Obito's body but it also gave him access to Hashirama Senju's
 abilities, the wood release ability, as seen when he defeated the
 ninjas who wanted to steal Rin's body.
 Now Hashirama Senju had unique medical ninjutsu which healed himself without forming any
 seals whatsoever. So if Obito gained access to the wood release ninjutsu he may also gained 
 this medical ninjutsu, which in turn healed his eye throughout the years.