  Estimating black hole masses of blazars is still a big challenge. Because of
the contamination of jets, using the previously suggested size -- continuum
luminosity relation can overestimate the broad line region (BLR) size and black
hole mass for radio-loud AGNs, including blazars. We propose a new relation
between the BLR size and $H_{\beta}$ emission line luminosity and present
evidences for using it to get more accurate black hole masses of radio-loud
AGNs. For extremely radio-loud AGNs such as blazars with weak/absent emission
lines, we suggest to use the fundamental plane relation of their elliptical
host galaxies to estimate the central velocity dispersions and black hole
masses, if their velocity dispersions are not known but the host galaxies can
be mapped. The black hole masses of some well-known blazars, such as OJ 287, AO
0235+164 and 3C 66B, are obtained using these two methods and the M - $\sigma$
relation. The implications of their black hole masses on other related studies
are also discussed.
