the.com/black market

means An illegal system of trade where goods or currency are bought and sold outside official, government-regulated channels, usually to dodge taxes, rationing, or bans.

from The phrase surfaces in English in the early 20th century, gaining real traction during the world wars, when rationing made ordinary goods scarce and official prices meaningless. 'Black' had long carried associations with the illicit, concealed, and sinister (blacklist, blackmail, black sheep), so pairing it with 'market' simply named the shadow economy that ran alongside the legal one — transactions done in the dark, off the books, away from the regulated stalls.