I've spent some time looking for a material to cover two extremely large bass traps (over 10' x 10' of angled wall). I was going to go with burlap, but you can't get it in 10'+ wide strips, and I didn't want seams to show.
I finally found a decent black muslin material, available 126" wide in any number of yards. It's used in backdrops on stages, but passes enough air to be used on traps - seems to be roughly equivalent to burlap, sonically. It's a very flat deep black, not at all shiny, so it's good in video installations. It runs around 10$/yard, though.
www.iweiss.commichelleb@iweiss.com