From THTF P. 301: Chapter 9, "Meanings"

BLIND SPOT: That would be only if he had one eye out, or a bad eye. Then it would be harder for him to see things on the side that didn’t have a good eye and — depending on where he is — his self-preservation would cause him to operate a little different to make up for that eye that has the problem. But if a horse has two good eyes, why, there isn’t any blind spot because he will move around so he can see something if he wants to. But people speak about it anyway