A black market is what tends to happen when goods and services are made illegal. The black market is economic activity that is illegal. Because black markets exist outside of government oversight and control, other than legal costs and the costs of avoiding detection they are Laissez-Faire economics.
Drugs and alcohol
In the United States, the drug war has failed horribly. About 119 million Americans have used an illicit substance at least once, and an estimated 22 million Americans used an illegal drug at least once a month in 2009.  The War on Drugs along with lobbying by private prison industry have led to a massive prison population, and has led to drug violence in
Mexico many parts of the world. The prohibition of Alcohol in the United States, which lasted from 1920 to 1933, was also a failure. A black market arose for alcohol. This illegal alcohol was often very toxic, and was sometimes lethal. Today, the drinking age in the United States is 21. Many teenagers start drinking long before they are legally allowed.
If gun control is put in place, it is likely that it would lead to a black market, like what seems to have happened for everything else that was made illegal. There could be a violent trade of guns, like what has happened with the Drug War. On the other hand, lax gun control in the United States makes it very easy for drug cartels to acquire guns, which they use in gang violence. Lax gun control also makes it easy for mass shooters to acquire weapons. However, readily accessible guns where mass murderers strike may make it easier for mass shooters to be stopped. It is uncertain how the level of violence with gun control will compare to the level of violence without it.
All other industrial countries have tighter gun controls than the United States. It's unlikely that the USA is right and everyone else is wrong.
In places where abortion is banned or severely restricted, it too leads to a black market. Normally, abortion is one of the safest medical procedures. Abortions that are done illegally are often dangerous, sometimes leading to maternal deaths and disabilities.
Prostitution is illegal in all states except for Nevada, yet prostitution still happens in states where it is illegal. Laws banning prostitution are largely unenforceable. Prostitution and brothels that are part of the black market cannot be regulated, so people who have sex with these prostitutes raise their risk of contracting STD's. Legal brothels can be regulated.