Ahahaha, OH MY... I should've checked here again, it's been a while. Yes, <damn> I'm busted. Nah, it's been an experiment that still works fantastic, but requires a small change in the GLSL shader code. I didn't think it was...uhm....something I should impose on the community, while it is incredible. It allows you to have full color diffuse and specular (reflection) shading. I've used it a few times. I wanted to add a mode to MaCrea that allows it to spit out those materials as well. Ah well...
Anyway, black is literally black and acts as the shadow color. I've later changed it so that you'd do your ordinary texturing and material painting and just add a black material at the end, loading in the inverted ambient occlusion map as texture for it. Works beautifully.
You could use something else than black, too, if you had some different idea about your shadow area color, but the idea was to let it go completely lightless, if that was the case.
THANKS, by the way, I'm always happy to hear that. If somebody was too keen on figuring out my dual material sphere stuff, well... I'll let you in on it, of course. It's quite magical, though. Opens up a different world of realism within sculptris!