This was a US contingency plan within the 'Color-coded War Plan' scheme for implementation in the event of war with Germany (1920s/30s).
The best-known version of 'Black' had its origins in World War I in case France fell and the Germans attempted to seize French possessions in the Caribbean, or launch an attack on the eastern seaboard of the USA. The US Navy was to lay mines and have submarines on patrol at sites Germany might seize for a foothold in the Caribbean. The plan was revised in 1916 to concentrate the main US naval fleet in New England, and from there defend the USA from the German navy. After Germany’s defeat in 1918, the plan lost importance.