I have been looking at French Paper Company for dark paper.
There is a rich dark blue, and a pure black under the 'Construction-Tones' (They have funny names for all of their collections.) 50 sheets of 80lb Cover, size 12.5" x 19" is about $28. This would be fine for Letterpress printing, relief block printing. I'm not sure about silk screen - it might curl.
It is not 100% cotton paper, so it is not forever archival, but I think it would be ok. They have many other colors too, if people are interested."
I'm thinking of using Rives Lightweight, which is the same weight: 115 gsm, 80 lbs/1000
You don't have to have your working proof printed on your final paper if you are still researching and testing papers to find out what will work best for your print. If you have comments or questions let's discuss them on this blog!