Why are black dwarfs virtually invisible?
A black dwarf is a white dwarf that has cooled to the point of emitting very little radiation. Add to that its small size (roughly the same size as the Earth) and you have a small object that is only really visible by the gravitational effects it has on nearby objects and the effect of transits.
They don't exist - at least not yet. The expected time for a white dwarf to cool and become a black dwarf is about