noun
Refers to an illegal market, especially those that flourished in post-war Japan.
戦後、闇市で食料を手に入れた。
After the war, people got food on the black market.
闇取引 refers to an illegal transaction or deal, while 闇市 is the physical or conceptual marketplace itself.
Compound of 闇 (yami, 'darkness; illegal') and 市 (ichi, 'market').