In a normal economy, currency has value because it's accepted everywhere for local transactions because of certain safeguards*. That is, if I hand over money to a store to buy something, and the storekeeper never hands me my item, I can go and ask Plod for help. And the converse situation holds as well. In a black market, you have to rely on word of mouth (or force) to put the unscrupulous out of business.
(* The flip side is, as Libertarians are fond of pointing out, is the currency can be manipulated. That's the price you pay for having the state involved in maintaining the Rule of Law.)