By nature of black holes: we don't know. For us it is pretty much a mass removed from this universe. For all we know, it has the average density everywhere. But

By nature of black holes: we don't know. For us it is pretty much a mass removed from this universe. For all we know, it has the average density everywhere. But the currently prevalent theory posits the density of the volume of the black hole is actually zero, with all the mass in a single point (singularity).