Black markets are illegal markets that emerge in response to price controls. A few buyers are able to obtain the good at the open-market price; the rest must resort to illegal means. The additional demand is met by underground suppliers selling at much higher prices.
The government does not support the black market or any of their actions with getting items and selling them in other forms. Those who are in demand of a good when they have a hard time in getting it may purchase it illegally at a higher price just so they can receive that good. When there is an exchange of goods in the black market, these items are usually prohibited by the government and therefor illegally being sold.