Letterpress printing on black paper can pose challenges. Most letterpress inks are transparent, so they're not going to show up on a black background. One exception is silver. Silver ink is mostly opaque (about 80%), so if you're sneaky, you can print a silver ink run on the black paper to create a lighter base, then print any color you'd like on top.
For Alex Jackson Studio we started with thick 4-ply black museum board and a silver ink run. Then we added the red rose artwork and a subtle yellow stamen, leaving a super-thin silver outline and tiny silver veins. On top of the red, we printed a gray halftone gradation to create shading (check out our halftone blog post if you're not sure what that is).