A black dwarf is the remains of a Sun-sized star which has evolved to a white dwarf and subsequently cooled down such that it no longer gives out radiation. None are known to exist in our universe, as the time taken for a white dwarf to cool to such a degree is hypothesized to be longer than the lifespan of the universe to date.
Even at the epoch when black dwarfs exist they will be extremely difficult to detect, emitting thermal radiation at a temperature not much above that of the cosmic microwave background radiation. One of the only ways to detect them may be through their gravitational influence.
Black dwarfs should not be confused with brown dwarfs, which are formed when gas contracts to form a star, but does not possess enough mass to initiate and sustain hydrogen nuclear fusion. (NB: what we now refer to as "brown dwarfs" were at times called "black dwarfs" in the 1960s.)