Black operation is a covert operation by a government, government agency or a military organization. Covert operation means an intelligence operation so planned as to permit possible rejection by the sponsor. It involves often outside of standard military protocol, sometimes against law. The key feature of this operation is clandestine(kept secret).
These operation are carried out by FBI, CIA, MI6 and other intelligence services of other nations. The term black refers to secret in generic form. Sometimes they are totally hidden from the public.