I find that the cold makes a huge difference to whether I am susceptible to blackouts. When I am at a comfortable temperature, eg when I am doing dry statics, it is very unlikely that I will have any problems, even if I am going for a PB. When I am cold it is a different story. I had a major blackout + LMC, well within my limits, on a day when everything was going perfectly well up to that point, and the only thing that I think caused it was the cold. I was a normal colour when rescued, not seriously blue, so I couldn't have been too low on O2, and it was a time I had done literally hundreds of times in dry training without any trouble, about 1:00 less than my PB at the time.
I haven't heard much about this problem, but for me, the cold/blackout connection is ruining my freediving. It is something I am working on, and I will post updates on anything I discover.
Lucia