Complex sentence: Generally, black economies prosper when there are state restrictions on goods, such as rationing or prohibition of goods, as in The Prohibition period in the United States, when the Mafia rose on the back of the resulting black market activity in alcohol production and sales.
Split into simpler sentences: Generally, black economies prosper when there are state restrictions on goods. A good example is the Prohibition period in the United States, when the Mafia rose on the back of the resulting black market activity in banned alcohol production and sales.