This is the only coaster I have blacked out on.:
It was at Circus World in Haines City, FL. And called "Wiener Looping". I was shocked and a little bit disbelieving, so I rode it 4 times. I blacked out every time.
One way to fight blacking out due to G-Forces is to hold you breath and bare down like you are doing a abs crunch. Blacking out happens fairly regularly to fighter pilots and they have developed special suits called compression suits that prevent it from happening. What happens when you black out is the g-force causes too much blood to drain from your brain (down into your torso). In general it is not harmful.
The reports of blacking out on King Dominion's new Giga is WHY I want to go ride it.