I don't know... the seems brain to still fill the gap with whatever the surroundings are (invert the colors of your screen, and the blind spot is no longer grey).
another great thing to try with that demonstration is slide your finger on the screen up through that O. suddenly the tip of your finger disappears! then as you keep sliding it out of the blind spot, there is a grey are cutting your finger into 2 pieces! ... using a ruler you can measure exactly how big the blind spot is (it increases with distance)... all this is not reassuring at all: were a truck approaching you through that path, you would certainly not see it until it's too late (assuming your eye is fixed on something for some reason... and it was a really silent truck... and you were covering your other eye— still better chances than winning the lottery).
Does anyone know a blind person who was born blind? asking is the really only way to settle this. The closest I know is a friend of mine who's legally blind... but he can still see (poorly) out of one eye and he wasn't born that way either so it's no use asking him.— come to think about it I know two people who are blind out of one eye... three actually!: I remember my middle-school french teacher would go blind on one eye sometimes (I don't remember why; it was some sort of medical condition)... I guess I attract half-blind people?