I have blacked out, but not from flying.  I went right out to an unbelievable calmness and blackness, until I woke up.  LOL
Migraine aura I get.  It is the weirdest visual stuff in one's vision field and precursor to a 3 day migraine binge that is not fun.  After the 1st really bad episode, I found prescription meds not to get the migraine headache at least.  It is a frequency activity in the back part of the surface of the brain that like shimmers forward.  I actually enjoy watching the light show.
Playing Aces High and getting an ocular migraine aura, I actually can't see worth beans and everything blurs out for 30 minutes.  Then I am fine.  Once the aura starts, it plays itself through.  The frequency of the LCD screen does it.  I get auras more frequently playing Aces High.  If I am driving, I get off the road.  I worry about losing my driver's license.  
Worst G-load I ever did was that "Mission Mars" ride to Epcot.  It has actually killed some people.  But I did not black out, though I sure got pukey, and $5 a cup coke!  My doctor gave me the ok to do it before hand.  Now that I am over age 50, those amusement parks rides I just watch, these days.
Guys experiencing what HiTech has, really flying due to health issues, I hope you are smart enough to get the OK by doing a flight physical.  I know a couple buddies that stroked, they lost their pilot's license because of theirs.  I should have been on BP meds, and after mine, have ever since. 
From what I have read, blackout from flying is tunnel-vision.  That really does exist, I did it once on a bad drunk in my military years. LOL