Look to the right and hold your right hand at the very edge of your peripheral vision. Now turn your head to the left. Keep your right hand still, and then try to look at it by moving only your eyes, not your head. There isn't blackness there, you vision just doesn't go that far. You don't see, period. Being fully blind is having that complete absence of sensation in every direction.

That's how a blind person explained it to me. He also said to plug your nose and try to tell what you can smell, and that it's like that: there's nothing there and no sensation of smelling nothing, just like how a sighted person can't see X-rays.