Because no black market is a bad thing, of course. If the market has demand for hired killers, for example, obviously they should exist.
(The is/should fallacy of free marketism is legitimately scary to me)
The black market often brings goods in demand to people where the government blocks them. During many war times, they were the only way people got around rations to get food and luxuries in a decent supply.
Drugs are often a black market. Yet, by and large marijuana is harmless, legal alcohol is not.
Markets have existed before governments rose. Markets are not scary, they are what make us inherently human. The exchange.
Your fearmongering is rather sad. Yes, criminal acts are possible. That doesn't come from the nature of the exchange, it comes from human nature.