I don't get it: In the US (and probably the rest of the English speaking world,) "black market" means illegal goods: Illegal drugs, media piracy, stolen items, banned items, items that bypass taxes, illegal guns, ect.

These already exist, or existed: There's the pirate bay for piracy, and the various incantations of The Silk Road. These also attract a lot of attention and get shut down; so they need to operate in a weird "word of mouth" way where only the people who look for them find out about them.

Perhaps consider the term "flea market" or "art fair" instead? It implies legal goods sold by independent vendors; with little overhead from the venue.