Normally, the blind spot goes unnoticed, for two reasons. First, each eye fills in the gap in what the other eye is seeing. So, if a mosquito is hidden in the blind spot of your right eye, you can still detect it with your left. Each eye plays “backup” to the other.
So, does that mean that if you close one eye, you’ll see a blank “hole” in your vision, everywhere you look? While there really is a “hole” in your perception, it’s not that easy to detect. The reason is that the brain’s visual processing center fills in the “hole” by extrapolating from the surrounding area. So, if you look at a blue sky with one eye, the blind spot “sees” blue as well.
The way to “see” the blind spot is to both close one eye, and prevent your brain from extrapolating the surroundings. The way to do that is using an object that contrasts sharply with its surroundings but is small enough to “hide” inside the blind spot. In the example above, a round black spot is used.
The blind spot is one of my favorite topics for a classroom demonstration. Using a document camera, I project an “X” on the left side of the screen, and a “dot” a short distance to the right. I tell the entire class to close their left eye, focus their right eye on an “X” projected on the screen, and to raise their hand if the dot disappears. I then slide the dot slowly to the right (on the document camera, this is easy to do if the “X” and the “dot” are on separate pieces of paper). At first, only students in the front rows raise their hand, but as the visual angle between the shapes increases, hands go down in the front and start to go up further and further back in the room.