  The relation between accretion disc (thermal emission) and jet (non-thermal
emission) in blazars is still a mystery as, typically, the beamed jet emission
swamps the disc even in the ultraviolet band where disc emission peaks. In this
paper we estimate the accretion disc component for 136 flat-spectrum radio
quasars selected from the Deep X-ray Radio Blazar Survey. We do this by
deriving the accretion disc spectrum from the mass and accretion rate onto the
central black hole for each object, estimated using the emission line widths
and the power emitted from the broad line region. We find that non-thermal
emission dominates the optical/UV band of our sources. The thermal component,
in fact, is, on average, ~ 15 per cent of the total and > 90 per cent of the
objects in the sample have a thermal component < 0.5 of the total luminosity.
We then estimate the integrated disc and kinetic jet powers and find that, on
average, the disc luminosity is ~ 1 to 20 times the jet power (depending on the
uncertainties in the estimation of the latter quantity). A comparison with
previous, independent results favours a scenario in which jet and disk powers
are of the same order of magnitude. Extraction of energy from a rotating black
hole via the ``Blandford-Znajek'' mechanism fails to explain the estimated jet
power in the majority of our sources. Finally, we find that the typical masses
for our sources are ~ 5 10^8 solar masses and that, contrary to previous
claims, about one quarter of our radio quasars have relatively small (< 3 10^8
solar masses) black hole mass.
