  The observed relation between the X-ray and radio properties of
low-luminosity accreting black holes has enabled the identification of multiple
candidate black hole X-ray binaries (BHXBs) in globular clusters. Here we
report an identification of the radio source VLA J213002.08+120904 (aka M15
S2), recently reported in Kirsten et al. 2014, as a BHXB candidate. They showed
that the parallax of this flat-spectrum variable radio source indicates a
2.2$^{+0.5}_{-0.3}$ kpc distance, which identifies it as lying in the
foreground of the globular cluster M15. We determine the radio characteristics
of this source, and place a deep limit on the X-ray luminosity of
$\sim4\times10^{29}$ erg s$^{-1}$. Furthermore, we astrometrically identify a
faint red stellar counterpart in archival Hubble images, with colors consistent
with a foreground star; at 2.2 kpc its inferred mass is 0.1-0.2 $M_{\odot}$. We
rule out that this object is a pulsar, neutron star X-ray binary, cataclysmic
variable, or planetary nebula, concluding that VLA J213002.08+120904 is the
first accreting black hole X-ray binary candidate discovered in quiescence
outside a globular cluster. Given the relatively small area over which parallax
studies of radio sources have been performed, this discovery suggests a much
larger population of quiescent BHXBs in our Galaxy,
$2.6\times10^4-1.7\times10^8$ BHXBs at $3\sigma$ confidence, than has been
previously estimated ($\sim10^2-10^4$) through population synthesis.
