I have identified what it means when I have this blackout feeling of not being able to focus or even think. I feel like I am totally stupid, not able to put my thoughts together, and all I want to do is give up and just curl up on the floor.
It is kind of obvious now when I know it: it is exactly the CPTSD freeze reaction. It is a self defense to a situation of threat where I perceive no way to escape and I feel I would not win if I started to fight. Playing dead.
Recently this happened: someone was crossing my boundaries and I wasn't aware of it, only felt the increasing blackout feeling. They were asking me what is wrong and I could not tell. Their niceness made it harder for me to realise that they are also a perceived threat. Since I could not tell what was going on I started to think I am just screwed up and piece of crap. At one point they said my body is curled up in a self defense position. Wow. Yes it was. I didn't realise. That was it!
If your boundaries as a child are crossed by default what can you do? You can only curl up in this position and try not to think. Erase your mind. This is the answer that immediately comes to my mind when I ask myself this question, so it's very likely the same answer would have come when I was little. Erase your mind: so this is why I cannot focus when I am in this state. I can't put my thoughts together. I also do not have any thoughts about escaping the situation - and that is why I do not make the connection that the situation is the cause of my distress! I even feel like staying in it is necessary at the moment, sometimes till another person intervenes. This is playing dead.
Conclusion: a blackout feeling means I am being triggered. Means I am in a situation where I feel something I don't want is imposed on me and I think I have no choice. In such situation I should check my boundaries. I should check where I have no choice and question that, because this is almost never the case.