Take a UGC, paint it black, presto black UGC.
Believe it or not I've already considered this. I have a can of black Krylon Fusion spray paint that I used to modify the color of my 360 and controller, it looks very smooth and holds up strong, the only issue is the plastic sleeve. There's no way to successfully paint around it and the final result not look bad, since there's not going to be a way to cover up the "missed" area of the case.
Actually just wrap the sleeve in few times in news paper, close the case and paint.
the parts under the sleeve don't have to be painted as they won't show.
or! couldn't you just paint the
inside of the case? since they're clear, the black would show through to the outside. then you don't have to worry about the sleeve or anything. hopefully the paint wouldn't rub off on any games or manuals though

all in all though, it seems like the easiest thing to do would just be to buy black 28mm dvd cases. sure, they don't hold the games precisely, but.. big deal! plus you can get them in other colors too.