You did what?
The silvery finish that was typical
back when your Pistol was made was
Matte Industrial Hard Chrome. You may have thought you were polishing Matte Stainless Steel, but I seriously doubt that is the case. Now there were a limited number of Stainless Witnesses made during the very early 1990's, but I seriously doubt that your Pistol is Stainless.
IF your Pistol is not Stainless and IF all of the Matte Hard Chrome Finish has been removed, you might as well get it Nickeled or Blued, IF you want a shiny Pistol. Collector Value..............gone!
When Polishing, I've never had a problem with any Black Build-up, but you might try Charcoal Lighter Fluid and see if that will take it off.
This reminds me of how I got my original Canadian Import Pre-B CZ75. Guy comes into the Gun Shop I worked in with several Rifles, Shotguns, and one Handgun. Guy had come back from a long fishing trip to find that his Wife had cleaned all of his Firearms for him, removing all of that ugly Black Stuff that was all over them.
To help pay for the Refinishing of the Long Arms, he traded me that Pre-B CZ75 and the rest shall we say is history. Had to sell it off a few years back and sometimes miss it.