Depends on what you consider hitting a black hole.

Generally, if you mean hitting the event horizon, no energy is released. The gravitational pull is so powerful that the energy never leaves the center of the black hole.

Now, if you're talking about the accretion disk, that's something totally different.

If a particle hits the surrounding, spinning gas and the gas disk is dense enough, the friction releases heat energy which can still escape since it isn't at the horizon, so it can escape in the form of x-rays or even gamma rays.

You might have gotten the 10% energy release from accretion striking the surface of a neutron star (but don't quote me on that). Just curious where you got that figure because a black hole's mass-energy conversion runs at around 43% making it the most efficient generator in the universe (taken that you can harness that energy).

-Vince

(Remember some of this stuff because I just finished a project on black holes.)

• Event Horizon: Shedding Light on Black Holes