The Black Market was part of an upcoming update for Miner's Haven, including a new map.
The Masked Man ended up replacing the Black Market and is active 8:00 PM EST Thursday - 8:00 PM EST Sunday, selling 4 contraband or exotic items every day. Because of this, the Black Market has been scrapped.
When it still existed, the Black Market was a large rock accessibleby a water tube hidden under the ocean. This was when you could swim in the water, instead of just running on the bottom. This rock never actually did anything.