This page seems to think underground market is a synonym for black market: https://www.thefreedictionary.com/Underground+market https://www.thefreedictionary.c

This page seems to think underground market is a synonym for black market: https://www.thefreedictionary.com/Underground+market https://www.thefreedictionary.com/Underground+market Wikipedia also uses the phrase "underground market" seemingly interchangeably with "black market": > Even when the underground market offers lower prices, consumers still have an incentive to buy on the legal market when possible, because: https://en.wikipedia.org/wiki/Black_market https://en.wikipedia.org/wiki/Black_market But you could also just say "illegal market" or just "slave market".