Black Market is a shop where various heroes visit everyday.
There can be 3 heroes in the Black market at any given time, each staying for 24 hours. Any hero can visit you, however if you have the given hero upgraded to certain star levels, you will have access to more trade options from that hero.
Each hero has a pool of different items and resources that can appear in the market, each in exchange for specific items and resources of interest of that hero.
You have the option to change the visiting hero for gold or by watching an ad - you cannot refresh to any of the last 10 heroes that visited you.
There are various ways to obtain Black Market Tokens, including completing Black Market trades using gold. You can exchange them for additional items within this market.