The optical counterpart of the black-hole soft X-ray transient Nova Muscae 1991 has brightened by \Delta { V } \approx 0.8 mag since its return to quiescence 23 years ago . We present the first clear evidence that the brightening of soft X-ray transients in quiescence occurs at a nearly linear rate . This discovery , and our precise determination of the disk component of emission obtained using our simultaneous photometric and spectroscopic data , have allowed us to identify and accurately model archival ellipsoidal light curves of the highest quality . The simultaneity , and the strong constraint it provides on the component of disk emission , is a key element of our work . Based on our analysis of the light curves , and our earlier measurements of the mass function and mass ratio , we have obtained for Nova Muscae 1991 the first accurate estimates of its systemic inclination i = 43.2 ^ { +2.1 } _ { -2.7 } deg , and black hole mass M = 11.0 ^ { +2.1 } _ { -1.4 } M _ { \odot } . Based on our determination of the radius of the secondary , we estimate the distance to be D = 4.95 ^ { +0.69 } _ { -0.65 } kpc . We discuss the implications of our work for future dynamical studies of black-hole soft X-ray transients .