So after a second round of retrobrite and some thorough cleaning, I think I was a bit off on what happened - the black part of the sticker didn't bleed through, it actiqlly protected the plastic underneath so well that it's the original, perfect, pristine color, and even after all sorts of treatment I guess I'll never get the rest of the case to quite match it. It's a little better after the second round of treatment at least...