  In the presence of a magnetic field, rotational energy can be extracted from
black holes via the Blandford-Znajek mechanism. We use self-similar advection
dominated accretion (ADAF) models to estimate the efficiency of this mechanism
for black holes accreting from geometrically thick disks, in the light of
recent magnetohydrodynamic disk simulations, and show that the power from
electromagnetic energy extraction exceeds the accretion luminosity for ADAFs at
sufficiently low accretion rates. We consider the detectability of isolated
stellar mass black holes accreting from the ISM, and show that for any rapidly
rotating holes the efficiency of energy extraction could reach 0.01. The
estimated total luminosity would be consistent with the tentative
identification of some EGRET sources as accreting isolated black holes, if that
energy is radiated primarily as gamma rays. We discuss the importance of
emission from the Blandford-Znajek mechanism for the spectra of other advection
dominated accretion flows, especially those in low luminosity galactic nuclei.
