DEADLIEST COSMIC CATCH

As you know a black hole has no mass. The sigularity does. The rest of a black hole is simply made of the warping of space and time.
The common black hole (100 million in the Milky Way alone) spins.
In that spin there is immense energy. Galactic overlords have to 'mine' that energy for their craft reactors. It is very dangerous work.
Dirty black hole energy comes out the poles and is known as spinning gamma ray bursts. Pure massless black hole energy is just warping of space time but can not be removed past the event horizon.

Catching black hole energy is an art as much as a science. Just think of the drone magnetic traps having to travel near the speed of light and gradually slow down after dirty Black hole energy has been magnetically trapped. Remember it can only be extracted from the polar ejecta from black holes when a black hole can not contain all it is consuming. Call it cosmic vomit if you will.