Black core is probably a lot like the black center layer of a playing card. Pasteboard, the paper of a paper playing card, is two layers of paper with a layer in-between of glue that's been laced with graphite. The graphite makes the paper opaque. A poorly-made playing card sometimes has no graphite, or at least not enough graphite, resulting in paper you can see through under strong light, being able to read a reverse image of the face from the back.