If you don't want to mess with fabric coverings, you can buy a version of 703 that is faced with a black "skin". It's Owens Corning "Select Sound".
http://www.geocities.com/thatwascasey
I made 16 of those. One layer of regular 2" 703 plus a top layer of 2" Select Sound. Framed with MDF. Placed at all corner juctions and first reflection poins, they absorb down to about 60 Hz.