Which monsters are immune to Black Hole's vortex?
Not all bosses are affected. The damage is still applied, though.
You cannot move rooted monsters like worms and hives, these will report 'immune' when a Black Hole goes off near them.
Also, there is a "weight" assigned to monsters, that affects how much they move from things like knockback. Any ordinary monster above that threshhold will not be pulled toward the black hole, but still takes damage.