Plain black muslin. Where to buy?
I prefer duvateen to velvet. Velvet catches a sheen easily. Duvateens can be purchasd in almost any size through motion picture suppliers & better camera stores. One side is nappy & the other is similar to a muslin. A 12'x12' muslin will probably have two parrallel seams im it.
I would get a 12'x 12' or a 12'x20'. They are treated with flame retardant & it is the same fabric used on flags. There are grommet holes every foot or so.
Norms Studio Equipment, (818)766-6676, in LA makes their own duvateens & has very good prices. American Studio Equipment (818)768-8922 also makes their own & in addition, they make the world's best grip gear & stands.