  Despite the detection of a large number of gravitational microlensing events,
the nature of Galactic dark matter remains very uncertain. This uncertainty is
due to two major reasons: the lens parameter degeneracy in the measured
Einstein timescale and the blending problem in dense field photometry.
Recently, consideration has been given to routine astrometric followup
observations of lensing events using the {\it Space Interferometry Mission}
(SIM) as a means of breaking the lens parameter degeneracy in microlensing
events. In this paper, we show that in addition to breaking the lens parameter
degeneracy, SIM observations can also be used to correct for nearly all types
of blending. Therefore, by resolving both the problems of the lens parameter
degeneracy and blending, SIM observations of gravitational lensing events will
significantly better constrain the nature of Galactic dark matter.
