Let's say we have a reactor that taps into an artificial black hole (kugelblitz-type). What kind of matter would be needed to feed the black hole to sustain it? Would this matter be treated like fuel?
It doesn't particularly matter what type of material you throw into a black hole. About the only physical variables of a black hole that affect the outside universe are mass, rotation, and electric charge. There's no such thing as an 'iron' black hole or a 'hydrogen' black hole. In theory, if you're using a black hole for energy generation, you could use it as a perfectly efficient way to dispose of extraordinarily hazardous materials.
However, the kugelblitz poses a different problem altogether. While a black hole is perfectly happy eating raw sewage or radioactive waste, a kugelblitz-style black hole small enough to generate significant energy will have a diameter smaller than that of the atoms you're trying to cram into it - in fact, it will be smaller than the diameter of individual protons. Maybe if a proton hits it dead-on, the kugelblitz will effectively suck it in, but I don't know for sure. If that is the case, then your method of feeding the black hole would be to fire a steady stream of protons directly at it, which would mean that your fuel would be common hydrogen.
More likely, there is no practical means of feeding matter into a kugelblitz-style black hole, which means that the only way to 'recharge' it would be to replicate the conditions that created it: Using several seconds worth of the sun's total energy output to power a gargantuan laser array which focuses all that energy down to a single near-infinitesimally small point. With this in mind, kugelblitzes strike me as being a means of storing vast amounts of power, not producing vast amounts of power.
EDIT: It occurs to me that it might be easier to feed this micro-black hole than I initially thought. As I understand it, a black hole is, theoretically, actually a point of infinite density, and the size I was talking about is actually its event horizon - that is, the perimeter around the black hole where the escape velocity is equal to the speed of light. I mentioned earlier that an atom might be 'sucked' in if it touched the event horizon... and on reflection, that is exactly what would happen. I was thinking of the event horizon as being a hole too small for an atom to fit through, but the instant an atom touches the event horizon, it's doomed.
Also, the gravity of the black hole extends outside the event horizon, and while you wouldn't be able to feel it even standing a few meters away (at least for a black hole of, say, a million metric tons), it will serve to drag matter towards it. The black hole will be giving off a LOT of heat, and the combination of radiation pressure and the vaporization of any nearby matter will keep it from eating planets, but a stream of particles fired at the black hole fast enough to overcome the radiation pressure will hit a point where the gravity will start to overcome the radiation pressure, and be sucked in, requiring less insane accuracy on the part of the feeder. I'm not sure where that point is though, and I suspect that it's still smaller than the diameter of an atom.
Assuming this does work, though, a problem remains with the practicality of feeding a black hole with a single stream of atoms. By my (very rough) calculations, this IS doable, though you'll probably need at least a few dozen particle accelerators pointed at the black hole, firing a constant stream of relatively heavy atoms (iron, perhaps) at several percent of the speed of light.
Black holes have no hair, nor they are picky when it comes to feed them: any matter which is subject to gravitational attraction is welcome by them.
And since you are somehow tapping in the Hawking radiation to get energy produced by the black hole, yes, whatever matter you dump into it is basically a fuel.
Actually, it's very important what you feed the black hole. Yes, you can load whatever you want into a particle accelerator and throw it at the black hole but this will result in the eventual catastrophic destruction of your power station.
Remember, black holes have only three properties: mass, rotation, and charge. Throw atomic nuclei at it and it's going to get a positive charge. A big positive charge that will keep you from feeding it any more nuclei. Once you can't feed it you've got trouble.
The answer is you have to feed it neutrons, or else alternate feeding it hydrogen and electrons.
I beg to disagree with other answers. From black hole's perspective it is indeed doesn't matter what kind of matter it consumes. But if we want our experiment to be clean, we need to feed it exclusively with non-baryonic matter. Otherwise what we have won't be a "kugelblitz-type black hole" by definition.