A black market is an unlisted/unreported business that works around the law instead of your normal traditional means. Many Black markets are known for providing illegal goods and services such as prostitution, drugs, paintings, and weapons, but anything can be provided in a black market. Black markets are usually synonymous with seedy drug dealers and gangs. Many video games use black markets as a means of providing upgrades and weapons, usually to get around the barrier of legality.
Black markets in video games
Just Cause 2
In Just Cause 2 the infamous "Sloth demon" would work through the black market to drop off weapons and supplies via helicopter for Rico. Later on he will supply you with vehicles and upgrades for your weapons.
Grand Theft Auto 4
In Grand Theft Auto 4 you visit back alley shops from which guns and explosives are sold by a black market vendor. If you become good friends with Jacob, he sells you guns and supplies out of the back of his car for a discounted price.
In Wolfenstein you can buy upgrades from a small black market run by the brothers Anton and Stefan Kriege, both glad to take your stolen Nazi treasure.
Grand Theft Auto Chinatown Wars
In GTA Chinatown wars you can play a minigame labelled "Supply and Demand" where you buy illegal substances from a black market drug dealer to try and sell said substances at a higher price to another drug dealer.