Strong continuum gamma‐ray emission at ∼1 MeV possibly correlated with a narrow annihilation line at 511 keV has been observed from both Cygnus X‐1 and the Galactic Center. Such correlated emission has been interpreted as a unique gamma‐ray signature for theoretically predicted relativistic, positron‐electron pair‐dominated plasma in regions surrounding the black holes. In this paper, I review primarily the Cygnus X‐1 results, which have provided important new insights about the source. Cygnus X‐1 may be considered a canonical reference stellar black hole whose spectral and temporal characteristics can be used for comparison with those of other black‐hole candidates including the Galactic Center and AGN.