As I understand it, when the BAC gets high enough, the brain simply stops recording memories. When you get sober there's just a blank period. A brownout is when the BAC is high enough to mess with the production of memory, but not high enough to black it out. Your memory is just really hit or miss.
My first blackout scared me. I had written an entire page while drinking and the next day, when I found it, I had zero memory of it. I wish I had been scared straight right then and there!!!
When blackouts become routine, you're in trouble.