You can use Black dye for PR from US Composits.
I also use black Mica powder. But be sure it isn't black pearl. It's more of a really dark gray. It's nice, but not solid black.
BTW, brass will show through a black blank if it get too thin. Like on the cap of a Jr Gent 1. So don't skimp on the colorant, and I'd still back paint the blank.