Blackholes lose energy through “Hawking Radiation”, which is a surprising convergence of general relativity, quantum mechanics, and thermodynamics. Hawking also calculated what temperature a blackhole will appear to be radiating at. However, if the Sun ( kg) were a blackhole its temperature would be about 60 nK (nano Kelvin). Suddenly, you’ll find that most (as in “almost all”) of the entropy in the universe is tied up in blackholes. So, yes, blackholes will release all their energy, but you have to wait for the universe to cool down almost completely.