What does it mean when you stand up and stretch and everything goes black almost like you are momentarily blind and you feel dizzy so you have to hold onto a chair to keep your balance?

Answer 1

What you were experiencing, in doctor speak, is orthostatic hypotension, a sudden drop in blood pressure.

Normally, blood vessels constrict to maintain blood pressure and compensate for changes in position, but in some cases the blood vessels don't compensate and, when you stand up, the blood doesn't go to the brain. Basically, the reflexes of the body don't catch up with the blood vessels.