About 100 million black holes are estimated to exist in our Galaxy, a large fraction of which are expected to be isolated. Yet, not a single isolated black hole has been detected to date—all of the few dozen black holes detected in our Galaxy so far are in binaries. Furthermore, the measured masses of black holes in our Galaxy are inconsistent either with the theoretical expectations, or with the LIGO measurements of black hole masses in external galaxies. Mass determinations of even a small number of isolated black holes will provide important clues in our understanding of black hole properties, including their formation mechanism. Astrometric Microlensing is the only available technique capable of detecting isolated black holes and measuring their masses. Out of the hundreds of bulge microlensing events found annually by the OGLE and MOA surveys, a few are found to have very long durations (\(>200\) days), some of which are expected to be caused by isolated black holes. We have carried out high-precision astrometry of several such long-duration events using the WFC on HST. We will present new results from these HST programs aimed at the first detections of isolated stellar-mass black holes.