I made jewelry for a living, once. Paraphrasing Clint Eastwood, it wasn't much of a living.
I had a blackening solution that worked reliably on silver and gold alloys. I never tried it on base metal, but it likely would have worked. In fact, considering these were alloys incorporating base metals, perhaps it DID work. I may have a bottle somewhere, but.......
My point of posting is that it was REQUIRED to use a STEEL applicator. I would dip my STEEL probe into the bottle and "paint" what I wanted black. I would gently buff it and I could add another one or two coats to deepen the black. The finish was pretty black and pretty durable (rings and such, you know).
So, consider my emphasis on the word STEEL in considering how to apply blackening liquids. Or not.