A black market was a market consisting of all commerce on which applicable taxes and/or regulations of trade were being avoided. Items could be purchased on the black market by those not legally able to purchase them otherwise, often for a substantial fee.
Quark admitted that he ran a black market on Terok Nor during the Cardassian Occupation of Bajor. (DS9: "Necessary Evil") Ibudan used to run black market medical supplies through Terok Nor during the same period. (DS9: "A Man Alone")
Constable Odo sometimes prepared for an important operation by leaking word about it to the local black market. In 2369, Odo mentioned to Quark the upcoming shipment of deuridium arriving to Deep Space 9, much to the displeasure of Lieutenant Primmin of Starfleet Security. (DS9: "The Passenger")