Having mastered control over space-time, a super-civilization might want to fabricate their own black holes for a variety of tasks: waste disposal, starship propulsion, hyper-computing, maybe even time travel.
On the macro-scale, super-civilizations might re-engineer stars using black holes. The quest for immortality beyond a star's lifetime would be a big motivation.
Suppose extraterrestrials manufacture a black hole to accrete material from a burned-out star, a white dwarf. This would provide abundant energy beyond the star's fusion-burning stage.
But how would you prove the black hole was artificial? Tell-tale evidence would come from measurements that showed the black hole weighed less than 3.5 solar masses. That's the minimum mass for crushing matter into a black hole via a supernova core-collapse.
Some low-mass X-ray binary stellar pairs might be artificially constructed, Vidal speculates. Or, aliens might come up with ways to control natural X-ray binaries - just as our ancestors extracted energy from waterfalls.