  A strong hard X-ray luminosity from a blazar flags the presence of a very
powerful jet. If the jet power is in turn related to the mass accretion rate,
the most luminous hard X-ray blazars should pinpoint the largest accretion
rates, and therefore the largest black hole masses. These ideas are confirmed
by the Swift satellite observations of the blazar S5 0014+813, at the redshift
z=3.366. Swift detected this source with all its three instruments, from the
optical to the hard X-rays. Through the construction of its spectral energy
distribution we are confident that its optical-UV emission is thermal in
origin. Associating it to the emission of a standard optically thick
geometrically thin accretion disk, we find a black hole mass of 40 billion
solar masses, radiating at 40% the Eddington value. The derived mass is among
the largest ever found. Super-Eddington slim disks or thick disks with the
presence of a collimating funnel can in principle reduce the black hole mass
estimate, but tends to produce spectra bluer than observed.
