It IS capitalism. What are the minimal requirements for (public) capitalism to exist?
There are probably a million and one answers to that, but let me take a stab at it from a decidedly modern (from the period of the Industrial Revolution perhaps) and Euro-American perspective.
1. Private Property -- that is, the realm of possession that increases the wealth of the person(s) holding it, defended by force. Ownership of land, ownership of a place of production, ownership of an institution of monetary loan/investment. NOT a toothbrush.
2. Wage Labor -- that is, the extraction of a person's energy that's put to use creating a commodity or a service for a boss; the person is compensated with a wage rather than with the ability to decide how, when, why, and for whom that commodity or service is created/distributed. Further, whatever wealth is generated by that commodity or service goes exclusively to the owner of the place of production. NOT a voluntary arrangement.
3. Market Economy -- that is, a field where the exchange of goods and services are priced, negotiated, bought and sold. Profit is generated as the goods and services are sold for more than the cost of production (including maintenance/expansion of the place of production, wages, and other logistical expenditures). Profits go exclusively to the owner or owner's agent. NOT a voluntary arrangement.
4. A durable, value-laden, and enforced/defended Medium of Exchange -- that is, some form of money and/or credit, usually backed by some finite resource (gold, silver). Enforcement of the exclusive prerogative to mint the money rests with the institution capable of doing so: the state.
5. A worldview based on the scarcity of commodities and resources -- that is, one that fosters competition over that real or imagined scarcity.
A grey or black market requires scarcity, the exchange of goods and services in a market, and the use of a recognized (but not necessarily exclusive or enforced) medium of exchange. Sounds a lot like capitalism to me. While the goods and services may be illegal, the extraction of labor power from workers and the generation of wealth and profit from those who control the production and distribution of those goods and services are recognized as fully legitimate. The state doesn't really care as long as they get their cut; that's why gangsters are almost always indicted for tax avoidance and other related crimes.