Black dwarf is not a very popular name in astronomy, but it is usually reserved for the condition that a white dwarf gets into after cooling below a temperature of a few hundred degrees or less, so that it becomes 'black' at optical wavelengths. This takes 10s of billions of YEARS and there should not be many of these in today's young universe. They are degenerate matter supported by the so-called Fermi degeneracy pressure which prevents two electrons from occupying the same quantum state inside this compressed object.