The black market is the 'market' where goods and services are sold in a way that is illegal or for goods and services that are illegal. Obviously, it isn't a market in the physical sense of the word, but rather, a market meaning the collective transactions that take place between buyers and sellers.
There are some obvious goods and services that flow through their own black market: guns, drugs, some antiques and artifacts, and even human trafficking. But, there are also black markets for things that are legal to own, but buyers and sellers just want to avoid either high taxes or any record of them buying something.