Dumbledore doesn't realise it, but by colluding in Black's escape, he left himself wide open for blackmail. I never needed to do it, but had it been necessary, I could have let him know that either he did as he was told, or I'd be forced to "fess up to the Ministry about the whole thing. That would have been a last resort, and I'd have had to have some way to back up my memories in case Dumbledore Obliviated me, but I knew about Penseives before Harry-the-prat found out about them. They're in Hogwarts, a History, after all. Ah, well; I guess all that dreary studying came in handy for something besides being my two catspaws" insurance policy against academic failure.