A blind spot usually refers to one point in a field of vision that is obscured from view. In fact, each of our eyes has a tiny blind spot of its own due to the structure of the retina in the back of the eye.
Locating The Spot
To find your blind spot, use the two dots above.* Holding one hand over your left eye, look directly at the left-hand dot. Initially, you can see both dots even though you're looking directly at only one. But as you move closer to your monitor, the right-hand dot disappears.
If you move your eye, the dot will reappear, but as long as you focus on the first dot, the second will be invisible. Move even closer and the missing dot reappears.
The blind spot exists because of the structure of the vertebrate retina. The retina, on the back of the eye, is where the photoreceptor--or light sensitive--cells are located. These cells detect light and send the message to the brain. But in front of the photoreceptor cells, is a layer of nerves.
At one point, all these nerves come together and pass through the retina on their way to the brain. At that point on the retina, there are no photoreceptor cells--only a kind of hole for the nerves to pass through.
All To Do With Light
Light from the dot that you're focusing on strikes the center of your retina. Light from the other dot strikes the retina a little to one side.
As you move closer to your monitor screen, light from the other dot lands farther away on your retina and eventually strikes the blind spot where there are no photoreceptor cells. At that point the dot becomes invisible.
*If you prefer, you may put two black dots on a piece of paper about three inches apart. Then, rather than moving your head back and fourth, you can move the paper.