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).
This technique requires really tight registration, but it allows for an effect you don't see often with letterpress. Our PDX sports fan coasters were done the same way. More photos of Alex's cards are below.