When a star becomes a white dwarf it no longer has an internal heat source, instead staying luminous only because of its retained temperature. A black dwarf is a white dwarf that has given off all its heat and cooled down to the temperature of the surrounding universe, known as the cosmic microwave background, which is about 2.7 Kelvin.
While a white dwarf gives off heat via thermal radiation it does so incredibly slowly, because the densely packed electrons that prevent it from collapsing are excellent conductors of heat. A hypothetical white dwarf born at the start of the universe 13.7 billion years ago wouldn’t have cooled down to the temperature required to form a black dwarf even today. In fact, the estimated time for a white dwarf to cool to a black dwarf would be roughly 73,000 times the age of the universe. For this reason, unlike their white, red and brown dwarf cousins, black dwarfs have not yet been observed. Indeed, as a black dwarf would emit little to no radiation, finding one would be nigh on impossible, as it would appear almost invisible to us, other than the effects of its gravity.
Answered by HIW.