It’s most likely niello. There are formulas available in some books,
but it has a dangerous side. It’s best to find someone who has
experience with it and learn from them. I tried making niello and
selling it, but the best niello formulas aren’t necessarily the best
to produce in commercial quantity. Fischer’s in Pforzhiem sells
powdered niello, but I’ve never tried it.
Here’s a link to some of my niello pieces that are in the Orchid