I used to make meth on the stove. Not the oven. And there was never any black residue. But aside from that, I think anvil is right. It was probably just poor house keeping. If you are worried, tell her to get you a new one. Period. Tell her that you cant get it clean enough to use that keeps up with your sanitary needs. But I think you are safe from any residual build up. Have you tried a cleaner called awesome? I buy it at the dollar general store very cheap and it works better than 10 cleaners combined.