Your not alone in seeing black orbs, infact me and my sister see them occasionally. The ones I see are sort of black with grey/shadow colour or sometimes I see a blue mixed in, my sister sees black with purple. We didnt know what these were and we both started seeing them at the same time and other the last year or so ive got a pritty good idea what they are plus I asked a psychic friend who also agrees.
It appears to be negative energy which can be about for many different reasons. They tend to show up days after a particularly stressful time so can be negative thoughts if you like even if you feel fine or whoever they came from is over it. They can also show up after a cleansing or healing has taken place. I think the colour is perhaps the black orb being transmuted by the higher energy. These things do vanish so they clearly do not hang about and must disperse.
The black is showing the low vibration energy which is dense. Being able to actually see negative is very useful and empowering. You could do some space clearing if you see it again.