  Two independent groups reported the discovery of an isolated dark stellar
remnant in the microlensing event OGLE-2011-BLG-0462 based on photometric
ground-based observations coupled with astrometric measurements taken with the
Hubble Space Telescope. These two analyses yielded discrepant mass
measurements, with the first group reporting that the lensing object is a black
hole of 7.1 +/- 1.3 solar masses whereas the other concluded that the
microlensing event was caused by either a neutron star or a low-mass black hole
(1.6-4.4 solar masses). Here, we scrutinize the available photometric and
astrometric data and conclude that systematic errors are a cause of the
discrepant measurements. We find that the lens is an isolated black hole with a
mass of 7.88 +/- 0.82 solar masses located at a distance of 1.49 +/- 0.12 kpc.
We also study the impact of blending on the accuracy of astrometric
microlensing measurements. We find that low-level blending by source companions
is a major, previously unrecognized, challenge to astrometric microlensing
measurements of black hole masses.
