  Superradiant instability turns rotating astrophysical black holes into unique
probes of light axions. We consider what happens when a light axion is coupled
to a strongly coupled hidden gauge sector. In this case superradiance results
in an adiabatic increase of a hidden sector CP-violating $\theta$-parameter in
a near horizon region. This may trigger a first order phase transition in the
gauge sector. As a result a significant fraction of a black hole mass is
released as a cloud of hidden mesons and can be later converted into
electromagnetic radiation. This results in a violent electromagnetic burst. The
characteristic frequency of such bursts may range approximately from 100 eV to
100 MeV.
