Our black rhodium plating looks barely gray and nowhere near what
I've been seeing - true black.
I’ve found, using Rio’s pen plating version of black rhodium, that
you get the blackest color by putting on the bare minimum. It seems
that the longer you plate, the more it starts to get gray and
lighter and brighter. For the best black, use as thin a coating as
you can. I’m not sure why this is the case, and it may not be for all
black rhodium solutions, but at least for what I’ve used, with a
Rhodinette pen plater, that’s what I’ve seen. Also, pay attention to
the recommended voltages. Don’t go higher than specified for your
solution. Lower voltage may help. And be sure your articles are
properly electrocleaned before plating.