  Gravitational microlensing is a powerful tool to search for a population of
invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and
binary BHs at wide orbits that are complementary to gravitational wave
observations. By monitoring highly populated regions of source stars like the
MW bulge region, one can pursue microlensing events due to these BHs. We find
that if BHs have a Salpeter-like mass function extended beyond $30M_\odot$ and
a similar velocity and spatial structure to stars in the Galactic bulge and
disk regions, the BH population is a dominant source of the microlensing events
at long timescales of the microlensing light curve $\gtrsim 100~$days. This is
due to a boosted sensitivity of the microlensing event rate to lens mass, given
as $M^2$, for such long-timescale events. A monitoring observation of $2 \times
10^{10}$ stars in the bulge region over 10 years with the Rubin Observatory
Legacy Survey of Space and Time (LSST) would enable one to find about $6\times
10^5$ BH microlensing events. We evaluate the efficiency of potential LSST
cadences for characterizing the light curves of BH microlensing and find that
nearly all events of long timescales can be detected.
