Black smut on nickel

I tried stripping gold from mid-phos electroless nickel, using Technistrip AU gold stripper and ended up with black parts. I can't electrostrip because the base metal is copper. I can't find anything to remove this black smut. I tried cyanide, chromic acid, alkaline descale, acids, cathodic treatments, etc. The black stuff covers up as long as it's not exposed to air for long, but I have a bunch of parts that were. What is the black stuff and how can I remove it?

The black smut is probably Nickel sulfide and the way to remove it is to strip off the nickel from the copper. Also if this is immersion gold on top of nickel the gold immersion process could cause what the industry calls black pad.