For years, I have used of Velux blankets as backdrops, one dark green, one cream, and one pale blue. They live in a cabinet, folded, when they are not working, and before hanging they get a few minutes in the dryer set on "medium" to take out the wrinkles.

The blue one is really only useful for high-key where it is essentially blown out, since I can't seem to properly gauge the difference in B/W density between a light blue and skin tone--a medium brown blanket would be better, but Costco didn't carry those

Regarding muslin, my local Wal-Mart has it in 120" width (for quilt-making, I understand) and it is quite reasonably priced.