A black forum is a locked forum with a hidden registry. Only members in a user group that has access can post or view content in said forum. For all others the forum is essentially invisible. They will not see a listing for said forum, and were they to try and link through they would be stone walled or redirected be it back to a main page or to an unrelated site entirely.
The Black is probably a reference to Black Projects run by the military. Which are classified, and are not acknowledged to exist. For example Area 51 is a Black facility. The Stealth Bomber was a Black project.
I am actually somewhat amazed that so few of you are actually aware that such forums exist. Almost every site with forums has a black forum for use of staff. You do not see it, but you suspect that it is there. Ask the moderation staff here about their forum.
It wasn't from VG's black forum was it?
Proud Member of GAIBoWS (Gamers Against Irrational Bans of Weezy & Squilliam)