  We present the analysis of five black hole candidates identified from
gravitational microlensing surveys. Hubble Space Telescope astrometric data and
densely sampled lightcurves from ground-based microlensing surveys are fit with
a single-source, single-lens microlensing model in order to measure the mass
and luminosity of each lens and determine if it is a black hole. One of the
five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows
a significant $>1$ mas coherent astrometric shift, little to no lens flux, and
has an inferred lens mass of 1.6 - 4.4 $M_\odot$. This makes OB110462 the first
definitive discovery of a compact object through astrometric microlensing and
it is most likely either a neutron star or a low-mass black hole. This compact
object lens is relatively nearby (0.70-1.92 kpc) and has a slow transverse
motion of $<$30 km/s. OB110462 shows significant tension between models
well-fit to photometry vs. astrometry, making it currently difficult to
distinguish between a neutron star and a black hole. Additional observations
and modeling with more complex system geometries, such as binary sources are
needed to resolve the puzzling nature of this object. For the remaining four
candidates, the lens masses are $<2 M_\odot$ and they are unlikely to be black
holes; two of the four are likely white dwarfs or neutron stars. We compare the
full sample of five candidates to theoretical expectations on the number of
black holes in the Milky Way ($\sim 10^8$) and find reasonable agreement given
the small sample size.

Astrophysics of Galaxies;Solar and Stellar Astrophysics