the conventional definition of a market where taxes aren't paid (or other administrative requirements, eg certifications, inspections, etc) is "grey market"

it's only "black" if the goods are prohibited by laws that designate such trade as a felony.

i support both but grey is far more widespread in terms of total volume of money paid for the goods and services. grey markets appear in proportion with excessive regulations on markets, in the former soviet bloc it made up something like 80% of business. a friend of mine in serbia as a child in the 80s used to travel with his father across to the other side of the iron curtain, germany, france, england, etc to buy things like jeans and gaming consoles and suchlike, bring them back stashed in the boot and sell them to people in the area, no tax, no import duties, no license.

i'm very much against prohibition though. if the product does not violate the rights of any person the prohibition is really a monopoly (you can do it with a licence, eg, coca cola imports coca leaves to make the flavor, no other company is allowed to do this, this is totally overreaching on the idea of a trademark). for which reason i am grateful for the existence of the dark web. but postal systems have gone to shit in the years since silk road first appeared in almost all countries, unless they have deregulated courier services. the government monopoly on these is also fucking shit. only a few countries national postal services are still any good these days (pretty much australia and the uk)