A black market, underground economy, or shadow economy is a clandestine market or transaction that 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 (where prohibited), illegal currency transactions and human trafficking. Violations of the tax code involving income tax evasion constitutes membership in the unreported economy.

What's the best answer to this question: is it illegal to go on the black market?
yes