The brain "fills in" the missing information from the blind spot. Notice what you see in the location of the dot when it's in your blind spot. When the dot disappears, you do not perceive a hole of whiteness or blackness in its place; instead your brain invents a patch of the background pattern. Your brain, with no information from that particular spot in visual space, fills in with the patterns around it. You're not perceiving what's out there. You're perceiving whatever your brain tells you.