black holes lose mass by emitting hawking radiation through pair formation at the event horizon [a quantum physics thing]. this means nothing to 'big' [stellar size plus] black holes because they are cooler than the cosmic background radiation [i.e. they eat more than exercise]. for tiny black holes [around the mass of Mt Everest] that Hawking predicts should have formed shortly after the big bang, it is a whole different story. they are much 'hotter' than the background radiation [2.7K]. those that do not have a food supply would eventually lose enough mass to fall below the gravitational threshold necessary to remain black holes after a mere 10-15 billion years [right about now]. at that point, it is theorized, the black hole goes 'poof' and spectacularly releases its captive mass as a gamma ray burst. gamma ray bursts are, in fact, frequently observed, but, have thus far only been associated with supernovas

footnote: in theory, even 'big' black holes will eventually give up the ghost once the background radition drops to near absolute zero. it is, however, highly unlikely any of us will live long enough to observe them go 'poof'. nothing is forever, but, where galactic core sized black holes are concerned, it is as close as it gets.