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.
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