  We measure black hole masses for 71 BL Lac objects from the Sloan Digital Sky
Survey with redshifts out to z~0.4. We perform spectral decompositions of their
nuclei from their host galaxies and measure their stellar velocity dispersions.
Black hole masses are then derived from the black hole mass - stellar velocity
dispersion relation. We find BL Lac objects host black holes of similar masses,
~10^{8.5} M_sun, with a dispersion of 0.4 dex, similar to the uncertainties on
each black hole measurement. Therefore, all BL Lac objects in our sample have
the same indistinguishable black hole mass. These 71 BL Lac objects follow the
black hole mass - bulge luminosity relation, and their narrow range of host
galaxy luminosities confirm previous claims that BL Lac host galaxies can be
treated as standard candles. We conclude that the observed diversity in the
shapes of BL Lac object spectral energy distributions is not strongly driven by
black hole mass or host galaxy properties.
