One of the most effective materials of this kind are micro-fiber fleece blankets. In black they absorb light like a sponge, are not prone to wrinkle, weigh very little and can be rolled up compactly to put into a small duffel bag, and these blankets can be quite inexpensive.

Thanks - maybe a king size blanket will work. It has to be less expensive than some of the thin backgrounds I'v seen

Photek sells a "background in a bag" for that purpose:
http://www.photekusa.com/
#10 Blackest Black