The Black Market can be found at Social → Regional Market. The Black Market is a non-player character (NPC) that sells resources of every kind with a stock equivalent to one goblin's carry limit for every resource in the Regional Market.
The Black Market has been replaced by the Seller Dungeonville. Dungeonville only offers normal resources, not crafted resources, furniture, or Traps. The travel time is set to 30 minutes regardless of your location. (Verified 7/6/2012)