Hancock's of Paducah has a solid black 117" wide backing by Robert Kaufman at 12.98/yd
http://www.hancocks-paducah.com/Item--i-RK-INF-BK
and moda marble jet black 108" wide backing at 12.98/yd
http://www.hancocks-paducah.com/Item--i-M-11002-11
I have used both of these fabrics in 44" widths and, in fact, buy them both by the bolt (from this supplier). Moda marble is a heavenly soft fabric but is not truly solid black. It is marbled with gray. It works well with batiks, but you will lose some of the drama you would have with a true solid black contrast.
The Kaufman line (Kona cotton solids) is a heavier weight, more like broadcloth. It is quite nice and plays well with standard quilting cottons nonetheless. I would be happy to send you a sample of both.