It's fine, provided it's not a plot hole - i.e. your fantasy setting needs to not have abolished blindness as a realistic malady, which some settings do. E.g. LOTR 100% has blind people, while the Harry Potter universe only has very poor blind people, since solving blindness is as trivial as a polyjuice potion, even if nothing else works (and something more effective is bound to work).
It's fine, provided it's not a plot hole - i.e. your fantasy setting needs to not have abolished blindness as a realistic malady, which some settings do. E.g. LOTR 100% has blind people, while the Harry Potter universe only has very poor blind people, since solving blindness is as trivial as a polyjuice potion, even if nothing else works (and something more effective is bound to work).