The Black Team is not yet implemented.
The Black Team is a secondary, unreleased "rank" that a single team of players will be given in addition to their normal Ranked Play rank.
The plan is for The Black Team to be a "king of the mountain" style mechanic, that remains The Black Team until they are defeated. There are also plans for The Black Team to be continuously streamed to Twitch.
Original answer written by @ene on Discord