The characteristic feature of a black hole is that it represents a region of space … from which no form of matter, energy, or information can escape.
—Roger Penrose (1972). “Black Holes and Gravitational Theory”. Nature (236): 377—380.
"Black hole" was a nickname given to a number of anonymous and undisclosed spaceships during the Legacy era that were suspected of being operated by the Imperial Intelligence Service or the Imperial Special Operations Command. These spaceships were suspected of being of mobile prisons and torture facilities, where prisoners whose incarceration was illegal by Imperial law were taken. After the apprehension of these individuals, they were taken from their home worlds onto black hole vessels, where they were made to effectively disappear from the galaxy, a "forced disappearance" from their native worlds and communities. Upon these black holes, it was believed that Imperial Special Forces and Imperial Intelligence agents imprisoned high-value captives indefinitely, without right to a fair trial, and torturing them with extreme and illegal experimental methods, including amputation, and with interrogation droids, whose usage had been outlawed by Roan Fel under the "Victory Without War" public relations policy.
These operators of the Special Forces and Imperial Intelligence used these spaceships in order to illegally detain and to illegally torture prisoners, because since these vessels were constantly moving in deep space and executing random hyperspace jumps, it would be impossible for Imperial Mission Committee on Sentient Right Abuses officers or media crews to find them and to reveal their illegal programs. It also made it impossible for opposing forces to find members of their side who had been captured and taken aboard black holes, therefore allowing for the maximum security to detain high-value prisoners of war.
During the tenure of Grand Admiral Camila Ivy Tavira, the usage of "black holes" became routine, allowing Special Forces and Intelligence personnel to conduct extreme interrogations on prisoners without public knowledge of these immoral measures.
The moniker "black hole" was penned because cosmological black holes in real life were regions "from which no form of matter, energy, or information can escape". Likewise, once a prisoner had been taken aboard a black hole vessel, he or she effectively disappeared from the outside galaxy, fated to either die aboard the vessel or to live the rest of his or her mortal life on a vessel that was continually jumping through hyperspace.