There exists black plating solutions (Rio Grande) which I know
nothing about, except that the are there. If you use WinOx with a
steel applicator (as it says on the bottle), it will oxidize gold.
The caveat is that white golds are more difficult, and some are pretty
much impossible. You paint or dip the piece with solution, and
wherever you touch it with steel (an old file), it turns black or
grey. It's a bi-metallic thing, I guess......