Start with the real one, inside your eye. There is a point at the back of each eye where the nerve joins on, and that point cannot catch light, so a small piece of the world simply does not arrive. What makes it worth remembering is that it does not feel like a hole. The brain smooths the gap over, so the view looks whole and you never sense that anything is missing. Every other use of blind spot keeps both halves of that picture: one small patch is not being seen, and nothing warns you about it.