cafepress is in the process of allowing black tshirts now. I seen someone mention that they didn't. They just launched that feature this week. Sounds like you will be able to sell them very soon. (you can work on the designs now)
I make about $900 a month off of cafepress webmaster referrals alone, so the tshirts can't be all that bad. I ordered three of my own designs a month ago and they didn't turn out too bad. I have only washed them once so far though.
In case anyone wants to sign up:
Cafepress
