A "black background" refers to the absence of noise, allowing each instrument to exist apart from other instruments surrounding it, ALONG WITH its own pocket of the ambience of the venue. It generally means, as Ablang, put it, a lack of RFI/EMI, which add a sonic "halo" around the instruments, joining them together instead of displaying them as separate entitites (e.g., a violin section will sound like there are [depending on your tweeter, of course] 10 violins instead of, say, 5 or 6, or 3 or sometimes - 1 BIG violin. It will have liquidity, but not always, as some backgrounds are clear black, but dry-sounding (usually a lack of 4-8k airiness that allows the venue's ambience to distinguish itself as Carnegie, Musikverein, Avery Fisher, etc.)
The VPI TNT has a black, "velvety" background (the original one: I wouldn't know about later iterations), but it is not a liquid background, nor is it dry. It would be akin to a slightly humid background.
Otherwise, backgrounds are shades of gray-ishness, making for a less clear "picture" of the placement of instruments. Just think of it as a low, low, low, LOW noise floor.