Steve, I think the black might be a spray on the surface. Not sure how it would be applied.
I can't specifically remember what all colors have been seen inside. Been awhile. However, I definitely remember dark transparent colors. What comes to mind is dark amber and dark red. And I do for sure remember cateye-like structures being found inside.
My best guess is that they took dark based marbles, at least sometimes transparent, applied black to them in a manner similar to how iridescence is applied, and then applied the splatter layer over the black layer.