Wow. That’s a first. My inexpensive HF tumbler turns somethings black but that’s from the cheap tub breaking down I think. I tear everything down and clean it all and start fresh when that occurs.
I may try the semi-flat coke route just to see what happens.
I had some issues a while back with my tumbler, originally a rock tumbler with the black rubber barrels. I had pieces coming out really dark, and not just a coating in black stuff - the dark haze barely came off with sanding. It couldn’t be “just” rubber flakes off the inside of the barrel.
Eventually learned that steel has a chemical reaction with aluminum ions. Rock tumbling grit is often aluminum-oxide, and perhaps there was a residue of it left behind when the steel shot went in.
Everything got washed, I did try the flat cola trick, but I also built some new barrels to keep my various mediums completely separate. Haven’t had any troubles so far!
I had the same issue and asked Tony Davidson (@therealtonydavidson on Instagram - GREAT TUTORIALS!) she said it is often an issue with the water, switch to distilled water and add some Sunsheen Burnishing Fluid (dilute mixture).
Also be sure to keep your shot clean, I’ve used a dash of Bar Keepers Friend (find at any grocery or Walgreens, etc.) and water with the shot in a tupperware container instead of the tumbler, as it is abrasive, and I didn’t want to damage the barrel.