What is Black Market?

Legal Definition
A black market, underground economy, or shadow economy is a clandestine market or transaction which has some aspect of illegality or is characterized by some form of noncompliant behavior with an institutional set of rules. If the rule defines the set of goods and services whose production and distribution is prohibited by law, non-compliance with the rule constitutes a black market trade since the transaction itself is illegal. Parties engaging in the production or distribution of prohibited goods and services are members of the illegal economy. Examples include the drug trade, prostitution, illegal currency transactions and human trafficking. Violations of the tax code involving income tax evasion constitutes membership in the unreported economy. Because tax evasion or participation in a black market activity is illegal, participants will attempt to hide their behavior from the government or regulatory authority. Cash usage is the preferred medium of exchange in illegal transactions since cash usage does not leave a footprint. Common motives for operating in black markets are to trade contraband, avoid taxes and regulations, or skirt price controls or rationing. Typically the totality of such activity is referred to with the definite article as a complement to the official economies, by market for such goods and services, e.g. "the black market in bush meat".

The black market is distinct from the grey market, in which commodities are distributed through channels which, while legal, are unofficial, unauthorized, or unintended by the original manufacturer, and the white market.

Black money is the proceeds of an illegal transaction, on which income and other taxes have not been paid, and which can only be legitimised by some form of money laundering. Because of the clandestine nature of the black economy it is not possible to determine its size and scope.
-- Wikipedia
Legal Definition
An illegal market when consumer goods are not easy to find and taxed highly. Prices are either higher than controlled prices or lower due to tax evasion. Black markets have black money.