Did the sink look any better when you finished? Stainless steel is mixed with other things such as nickel and chromium and since hers had dents in it, I'm assuming it was a higher gauge and usually lower gauge has thicker steel. Usually BKF works really well in keeping a sink looking good, but I do remember that happening to one sink I had. I don't think black coming off is necessarily bad though. Builders even in high end houses often try to cut corners where consumers won't notice, or by the time they do, they won't hold him responsible.

Thanks for the reassurance. Yes, the sink did look better afterwards. I was just concerned about the black coming off. I have an 18 gauge Franke sink and that has never happened.