The Klingon Black Staff are an elite organization within the Klingon Empire who call themselves the "midshipmen for the Black Fleet". They essentially serve as a branch of the Klingon Defense Force who are assigned espionage missions that are of great concern to the Empire. They often have to deal with internal threats from subject races and, as such, often work with Imperial Intelligence.
Members of the Black Staff are feared by other Klingon captains and the bureaucrats of the subject races. Its members tend to be stoic who often abandon the comforts of family, friends and other pleasures in order to best serve the Empire.
Their name comes from the Black Fleet of Sto-Vo-Kor where Klingon commanders are called to serve in the afterlife. (TOS video game: Starfleet Command)