  Black hole superradiance is a powerful tool in the search for ultra-light
bosons. Constraints on the existence of such particles have been derived from
the observation of highly spinning black holes, absence of continuous
gravitational-wave signals, and of the associated stochastic background.
However, these constraints are only strictly speaking valid in the limit where
the boson's interactions can be neglected. In this work we investigate the
extent to which the superradiant growth of an ultra-light dark photon can be
quenched via scattering processes with ambient electrons. For dark photon
masses $m_{\gamma^\prime} \gtrsim 10^{-17}\,{\rm eV}$, and for reasonable
values of the ambient electron number density, we find superradiance can be
quenched prior to extracting a significant fraction of the black-hole spin. For
sufficiently large $m_{\gamma^\prime}$ and small electron number densities, the
in-medium suppression of the kinetic mixing can be efficiently removed, and
quenching occurs for mixings $\chi_0 \gtrsim \mathcal{O}(10^{-8})$; at low
masses, however, in-medium effects strongly inhibit otherwise efficient
scattering processes from dissipating energy. Intriguingly, this quenching
leads to a time- and energy-oscillating electromagnetic signature, with
luminosities potentially extending up to $\sim 10^{57}\,{\rm erg / s}$,
suggesting that such events should be detectable with existing telescopes. As a
byproduct we also show that superradiance cannot be used to constrain a small
mass for the Standard Model photon.
