The problem with most fabric you will find in a fabric store is the width...it usually maxes out at 60" wide (if that)...which might be wide enough for a single portrait but that's it. If you want wider, you would need to sew pieces together which leaves you with a seam. That's not the worst thing in the world but it's not good either.
If I was going to buy an actual backdrop, I'd want it to be at least 10 or 12 feet wide.
I honestly don't know where they get fabric that wide...I work for an industrial fabric company and the only stuff we have that is wider than 62" is a poly material that isn't really suitable as a backdrop.
With proper lighting (and control of that lighting) you can make any background appear black in a photo. Black velvet makes it easier though.