Blair didn't actually remember before it was too late. *SPOILERS!*
This topic is not about Blair being a sociopathic, two-timing monster hiding behind a sweet girl facade (this is something I agree with) but is instead about the ending. According to that website about "not answering calls from the dead," it states that in order to survive, the victim must confess their crimes. It seems to me that after seeing the ending, Blair actually forgot about being the one who filmed Laura (on a side note, she herself seemed a bit out of it/drunk after revealing herself to the camera) and just couldn't remember what she did in time in order to save her life. I think this is the case because as monstrous as Blair was to Laura and the others, she herself didn't seem to be the type to have others horribly die for the LOLs, especially when she knows that Laura is well aware of info that nobody else knows and seems genuinely scared by Laura killing her so-called "friends", thus it would be dumb of her to actually lie about something Laura already knows.
Also, if Blair really did remember herself filming Laura passed out drunk and soiled, she could've saved not only herself but the majority, if not the entirety, of the others when everything started going to Hell. Since Blair didn't confess to her crime even to save her own wretched skin after everybody was killed, it's a safe guess that Blair just forgot what she did and thus she and everybody else was doomed from the start. If that wasn't the case, then not only is Blair a sociopathic monster, but an extremely dumb one who can't even save her life. Either way, may she and Adam rest in Hell. The two of them were the worst of that group of "friends".
Welcome to my Nightmare- Freddy Krueger