Despite Black Holes not allowing the escape of energy or matter from themselves, this is only true for matter contained within the Schwarzschild radius. Any matter falling into a Black Hole will radiate energy up and until this radius is reached. As the body is accelerated ever faster as it ploughs towards the Black Hole, it radiates energy in the form of matter waves. If the body is also charged, it will radiate synchrotron radiation, gamma rays emitted by any accelerating charged body.

The efficiency of both processes reaches optimum when both bodies are of equal mass: two Black Holes accelerating towards each other and coalescing. The intense gravitational field around a Black Hole perturbs the space and time around it. As the Black Holes move through space, they leave behind a large wake of bent space-time waves, which travel away at light-speed. These are gravitational waves and for un-charged Black Holes can radiate away as much as 10% of the mass of each Black Hole. The mass has indeed somehow managed to escape from within the Schwarzschild radius, but this is no surprise, both are the same thing - gravity.

Fully, but oppositely, charged black holes can radiate away as much as 43% of their mass. Oppositely charged and oppositely spinning Black Holes can radiate away 90%?? of their mass. These processes can never be 100% efficient; a Black Hole or space-time singularity must always be present afterwards. See 'Nucleosynthesis by Neutron Star Mergers'

Except for matter/anti-matter annihilation, which is 100% efficient, these are by far and away the most efficient ways known of transforming energy from one form into another, and happen because gravitational energy is negative: when the Black Holes were apart they really wanted to be together; who separated them and with what energy? That is indeed another question!

See White Black Holes