  The ultraviolet (UV) spectra of 16 blazars (<z> ~1) from the archives of the
Hubble Space Telescope Faint Object Spectrograph have been analyzed in order to
study in a systematic way the properties oftheir broad UV emission lines. We
find that the luminosities of the most prominent and intense lines, Ly_alpha
and C IV lambda1549, are similar to those of normal radio-loud quasars at
comparable redshifts. However, the equivalent widths of blazar lines are
significantly smaller than those of radio-loud quasars. Therefore, while the
intrinsic broad line region luminosity of blazars appears to be
indistinguishable from that of radio-loud quasars, their continuum must be
comparatively higher, most probably due to relativistic beaming. We have
combined the UV luminosities of the de-beamed continuum with the emitting gas
velocity to derive estimates of the masses of the central supermassive black
holes. The size of the broad line region was computed in two ways: 1) via an
empirical relationship between UV continuum luminosity and broad line region
size, and 2) through the external photon density required by blazar models to
reproduce the inverse Compton components observed at gamma-rays. The second
method yields significantly different results from the first method, suggesting
that it provides only a very rough estimate or a lower limit on the size of the
broad line region. We find that the average mass of the central black holes in
blazars is ~2.8e8 Msun, with a large dispersion, comparable to those computed
for other radio-loud active galactic nuclei.
