  In viable models of minicharged dark matter, astrophysical black holes might
be charged under a hidden $U(1)$ symmetry and are formally described by the
same Kerr-Newman solution of Einstein-Maxwell theory. These objects are unique
probes of minicharged dark matter and dark photons. We show that the recent
gravitational-wave detection of a binary black-hole coalescence by aLIGO
provides various observational bounds on the black hole's charge, regardless of
its nature. The pre-merger inspiral phase can be used to constrain the dipolar
emission of (ordinary and dark) photons, whereas the detection of the
quasinormal modes set an upper limit on the final black hole's charge. By using
a toy model of a point charge plunging into a Reissner-Nordstrom black hole, we
also show that in dynamical processes the (hidden) electromagnetic quasinormal
modes of the final object are excited to considerable amplitude in the
gravitational-wave spectrum only when the black hole is nearly extremal. The
coalescence produces a burst of low-frequency dark photons which might provide
a possible electromagnetic counterpart to black-hole mergers in these
scenarios.
