I'm not someone so specialized about this, in case I say something stupid :)

This is the first time I've heard of the term "black market", but it sounds very interesting. How could it be implemented here? I find it a bit curious that when the trade is completed it becomes public, do you have any examples of an exchange that works that way? It reminds me a lot of p2p trades, something like localbitcoins.

Also, why not make a proposal in the DAO if it is a matter of getting STEEM liquidity? I think that way we could increase the liquidity of that black market if necessary.