Why are black dwarfs not on the hr diagram?
Because they are theoretical.
Black dwarfs are white dwarfs that have cooled down to the point that they blend in with the space around them, making no light or heat.
Because we can't see them, we don't know for sure if they really do exist. However, if they do exist and could be graphed, they would probably be in the lower-left hand corner, below brown dwarfs.