What would あなた do if あなた were Bellatrix and あなた suddenly found ot that あなた weren't really a pure-blood?
Probably freak out, kill anyone unfortunate enough to be nearby, go crucio some inoocent people, set 火災, 火 to any nearby buildings, laugh evilly, scream, destroy things, yell at people and calling them "filthy mudbloods", then possibly calm down a teeny bit. Repeat.
I think she would kill herself. Being pure-blood is everything to her so if she found out that she wasn't, what would the purpose of living be? Maybe kill everyone else in the world first :P