  Mixing between ultralight bosons and the Standard Model photon may allow
access to the hitherto invisible Universe. In the presence of plasma, photons
are dressed with an effective mass which will influence the conversion between
the two. We study this phenomenon, known as in-medium suppression, in the
context of black hole physics. We consider both axion-photon mixing around
charged black holes and dark photon-photon mixing around neutral black holes.
We find that the presence of plasma indeed influences the conversion rate,
possibly quenching it altogether for large plasma densities, and discuss
implications for superradiance and observational signatures.
