You noted yourself that government is in control of most current black markets. I have read this was also true in the USSR, where sugar, coffee, vodka, and etc. were rationed, but if you had the right connections in government you could buy on the black market.

There is a significant threat of governments controlling black markets, especially as surveillance becomes more intrusive. Therefore it is critically important to reduce surveillance, avoid dealing with people you don't know, and other opsec to enable secondary exchanges to prevent violent thugs - the government - from controlling these markets.

Thanks!