Hawking radiation. A black hole emits hawking radiation at a wavelength about equal to its diameter, and with power (ie energy/time) inversely proportional to its diameter. So the smaller the black hole, the higher the energy of the photons it emits and also the more energy it emits per second. A black hole with the mass of Mount Everest (~10^14 kg) would, according to a hawking radiation calculator[0], have a schwarzschild radius of 10^-23m, and a proportional wavelength on the order of extremely intense gamma rays or cosmic rays, with a temperature of a billion degrees kelvin. Low-mass black holes aren't black at all.

[0] Online hawking radiation counter: https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...

It's important to note that Hawking Radiation is not a confirmed phenomenon, and unless we actually find such a small black hole, it will never be confirmed (stellar mass black holes are too cold compared to the microwave background to emit Hawking radiation, and will continue being so for an extraordinarily long time.

Really it depends on what you consider a mass of an asteroid to be.

If it's large enough, the black hole is going to be releasing xrays. If it's smaller, it's going to be releasing gamma rays.

The smaller the black hole, the faster it evaporates. The faster it evaporates, the more energy it puts out.

The black hole at this scale is still going to have too small a cross section to suck up any matter and the radiation might end up blowing away any matter before it gets to the black hole in the first place.

At these masses, there's no real danger in the black hole passing through the Earth itself. The only danger is how much radiation is going to be shot out as it passes through the atmosphere on either side of the planet.

"Google-fu" :)