The Black Market is a place found through digging at certain locations while using Secret Maps.
These Maps are obtained through gathering weird map fragments at all locations with the exception of phantom school and providing them to the respective NPC's which will in return give you a Secret Maps for that location.
Once entering the Black Market, a range of exchange NPC's including Dark Dealder Rat, Compounder Vin, Trader Pav and Great Chef Pav are present allowing for low level equipment exchange, equipment upgrades and potions.