Various articles about black holes talk about X-rays emitted near the
event horizon 'reflecting off the accretion disc'.

What kind of material is it that can *reflect* X-rays? I've worked in
X-ray crystallography, and we needed grazing incidence off very
precisely figured monocrystalline silicon to get something that
reflected X-rays at 12.7keV (selenium K line); astrophysical X-rays
seem to be more at iron K which is about half that energy, but still
generally-occuring materials either absorb or transmit them.

Is this in fact more like the process around a nuclear detonation,
where things absorb X-rays and are themselves heated to X-ray-emitting
temperatures?

Tom

[[Mod. note -- Yes, thermal re-emission is one possibility.
Compton scattering is another possibility. As you note, coherent
reflection seems unlikely.
-- jt]]