I will relate my experience with a Wyandotte rooster that was prone to the same scary behavior.
The first time I saw this happen, I was just putting my chickens into the coop when he flapped his wings madly and fell off his perch. I grabbed him up, and like you, I panicked, not knowing what to do. He wasn't breathing and his comb was dark. I shook him and he regained consciousness.
Like a human with heart problems, the heart can stop, plunging the victim into a near death experience. A sudden shock or normal movement can start the heart pumping again. I gave my rooster a half a baby aspirin daily for a few months, and when nothing like that episode happened again, I stopped the aspirin. A few months later, I found him dead one morning under his perch.
I don't know for sure what his problem was. But some chickens hatch with genetic defects, and it's likely he had a heart with a congenital defect. Your rooster may also have such a defect.