If you cover your left eye and look with your right eye at one of the crosses, the circle directly to its right will disappear, due to the human blind spot.

Humans have blind spots, unlike many other animals, because we have veins in the backs of our eyes that block those respective lines of sight.

However, something I don't understand, why does the blind spot always disappear into the correct background color when you test it? In the black background example, the white circle disappears into black, and in the white background example, the black circle dissapears into white.

How does the human eye know how to fill in the gap in that way?