I'd highly recommend reading as much published information as you can about freediving and black outs.
A good place to start would be the article from last fall by Neal Pollock, PhD from DAN. It's on the Dive Wise website.
http://www.divewise.org/articles/pollock_bh_review.pdf
Julie has begun to collect blackout survival stories and memorials on the Dive Wise site. If you have a story others could learn from, please submit it.
Chad