  Contrary to a prevailing assumption that black holes would swiftly discharge,
we argue that black holes can charge preferentially when boosted through an
ambient magnetic field. Though the details are very different, the preference
for charge is related to the precipitation of the Wald charge on a spinning
black hole in an ambient magnetic field. The gravito-electrodynamics upstage
naive arguments about screening electric fields in determining the value of the
charge accrued. Charged test particles, which build up the black hole charge,
exhibit chaotic behavior as evidenced by fractal basin boundaries between
dynamical regions. Charged, boosted black holes will generate their own
electromagnetic fields and thereby their own luminous signatures, even if they
are initially bare. We therefore add boosted black holes to the growing list of
potentially observable black hole signatures, alongside black hole batteries
and black hole pulsars. The implications should be relevant for supermassive
black holes that are boosted relative to a galactic magnetic field as well as
black holes merging with magnetized neutron stars.
