Jim M,
You can use "Brass Black" (sold just for blackening brass) after the engraving is done and then polish the surface with some 1500 or 2000 grit automotive polishing paper with a backing block. Or you can use black lacquer paint (which dries very quickly) and, just as it gets tacky, wipe the surface clean with thinner. Or, you can use printers ink. It takes a long time to dry, but if you buff it off the surface well, it will dry over the next few days down in the engraving without much trouble.