Black Dispatches was a common term used among Union military men in the American Civil War for intelligence on Confederate forces provided by Negroes. This source of information represented a prolific and productive category of intelligence obtained and acted on by Union forces throughout the Civil War.
Black Dispatches resulted from frontline tactical debriefings of slaves--either runaways or those having just come under Union control. Black Americans also contributed, however, to tactical and strategic Union intelligence through behind-the-lines missions and agent-in-place operations. Two such Union agents functioned as long-term penetrations of Confederate President Jefferson Davis's "White House" staff in Richmond, Virginia.wiki
Union officers got so many valuable pieces of intelligence from slaves that the reports were put in a special category: "Black Dispatches." Runaway slaves, many of them conscripted to work on Confederate fortifications, gave the Union Army a continually flowing stream of intelligence. So did slaves who volunteered to be stay-in-place agents.cia.gov