Black team members are the sneaky guys, they do illegal activities but like to hide in the shadows while they do it. In game terms, certain things that has a negative effect on the BGS belongs to the back team.
The following tasks are considered black team tasks:
Pirating (attacking traders, stealing their cargo and selling it on the black market)
Illegal trading (selling illegal commodities on the black market)
UA bombing (picking up unidentified artifacts and selling them at the black market)