The portal is open. The Black Gate is now open in Raid Finder difficulty! Blizzard announced yesterday that the final wing of Hellfire Citadel, the Black Gate, is now open for business on the raid finder.
The Black Gate is a Dungeons & Raids achievement earned for defeating Archimonde in Hellfire Citadel. Hellfire Citadel opened for business on June 23rd and has been gradually unlocking difficulties and raid finder options over the course of the summer.
The Raid Finder, accessed through the Group Finder, is a tool that helps players find and join a random or specific raid instance with others. The feature was first released in patch 4.3.0 for Dragon Soul as a permanent addition to the raiding system, and has been included in all subsequent instanced raids.