A list of currently known black holes. Although you can't see black holes, it is nonetheless interesting to know which constellations they are found in.

Black holes (BHs) are formed by the gravitational core collapse of massive stars during a Type II supernova. As such, they are dead (degenerate) because they no longer generate fusion. Most galaxies in the universe have a supermassive black hole at their center. And there are two trillion galaxies in the observable universe. As such, this list only includes the smaller black holes thus far found in our galaxy, with a couple of notable exceptions:

Sgr A* - supermassive black hole at the center of our Milky Way galaxy
M87* - supermassive black hole at the center of the Virgo A galaxy (first BH imaged)

HR 6819 is actually a black hole and two stars forming a trinary system. These stars are visible to the naked eye.