The thing with aspects like that is that the obvious effect of the aspect only applies when it is relevant to the story i.e. the action. Technically they are BLIND all the time but it only matters when someone makes it matter, by tagging or compelling the aspect. The two main ways to use BLIND that I can see off-hand is a) tag to get an advantage - "he can't see, so he can't stop me hitting him!" - to get +2, or b) compel them so they stumble around for a bit and can't act - "your character is BLIND so he can't see: what does he do?"
This is easily the most counter-intuitive part of FATE by a long shot!