I actually recently summoned a black orb, consciously without a clue about what I was doing, subconsciously with full intent and knowledge of exactly what I was doing. Confusing but this what happens when your subconscious wants to do something beyond what your conscious mind can remember.

Firstly, Black is not a 'dark' or 'evil' color.. it can actually be quite purifying and I have even used it in healing before. However much of the Dark does like to adopt the blackness fora myriad of other reasons including hiding their true identity.

Spirits may travel in orb form quite frequently, some most resonate with the color black and therefore appear black. In saying this, I don't trust regular spirits good/bad and get rid of them whenever I see them anyway. Others are more lenient towards the spirit world.

Black orbs were also used in the way old ancient days as constructs of twin energies. A twin energy to follow you into war, etc.