so, it confirms two things: the amount and the truth of if it spent it in the first place?

so it is a simple form of zero knowledge proof with one small associated value, ie, the amount, plus one bit of truth

well, if that's exactly how it works, and i believe it could be, i never read deep into blinded signatures, then very cool, and that also means chaumian mints act as perfect mixers, because what goes in cannot be traced to what goes out in as far as the amounts differ