There will often be some dark stains left behind, either from spots where rust was once active or from a stubborn carbon wart, and that's not unusual. Again, don't use anything harsh to try to remove them or you risk damaging metal. And by damage, I mean the leaving of any kind of marks, either scraping, grinding, or even polishing swirl marks. The consensus among collectors, though, is it's normal to have some staining and cooking utensil marks on vintage cast iron because people didn't buy pans back-when to display, they bought them to use. You can try to remove or at least minimize residual stains by soaking the piece in a solution of 50/50 white vinegar and water for 30 minutes at a time, followed by a buffing with 0000 steel wool. Rinse thoroughly with plain water to neutralize the vinegar solution.
Never leave a piece in the vinegar solution for longer than 30 minutes.
An alternate method is to wet the piece and then apply white vinegar full strength with a sprayer bottle. Let stand a few minutes and scrub with a stainless steel scrubber or steel wool, again followed by a thorough rinsing.
Never leave a piece with full strength vinegar on it for longer than a few minutes.
With repeated cooking use, however, layers of new seasoning will continue to darken the piece, and any stains that did not yield to the vinegar treatment will blend in.