Definition: Black Market Premium

Black Market Premium - "the extent to which the illegal or unofficial market price of a good or currency exceeds the official market price. For example, if Ethiopia’s official exchange rate is Birr9 = US$1 and the black market rate is Birr18 = US$1, then the premium is 100 percent (18–9/9 = 1.00)." - source: Economic Development 4th Edition by E. Wayne Nafziger