Why there's nothing there
This is where the optic nerve leaves the eye. There are no light-sensitive cells there, so no image comes from that spot. It lies about 15 degrees to the side of the point you're looking at.
Your brain fills it in
You never notice the hole because your brain fills it: with what's around it and with the image from the other eye. Switch the exercise to "Line with gap" – in the blind spot your brain simply closes the gap.
The octopus doesn't have one
In vertebrates the nerve fibers run in front of the retina and have to pass through it somewhere. In the octopus they lie behind it. Its eye, which evolved independently of ours, therefore has no blind spot.
