A quick search of black + low voltage enclosure shows Leviton makes one.
I expected Panduit to have one - quick review of their website shows no standard black enclosures.
An auto body shop could paint any steel enclosure black, and the finish would be more durable than if done with ordinary black spray paint (which is another option). Bring a blank rack plate to the shop, to match the finish (wide variety of black finish options).
You could also just paint the door of the enclosure; nobody but you will see the interior of the LV enclosure. Most LV enclosure doors cover the whole front of the enclosure, although YMMV
Edit - paint cover only if recessed/flush mount; paint enclosure too if surface-mount. Edited by Neurorad - 4/22/13 at 12:33pm