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 thus 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-ultraviolet (UV) emission is thermal in origin. Associating it with the emission of a standard optically thick geometrically thin accretion disc, we find a black hole mass, M∼ 4 × 1010 M⊙, radiating at 40 per cent the Eddington value. The derived mass is among the largest ever found. Super-Eddington slim discs or thick discs with the presence of a collimating funnel can in principle reduce the black hole mass estimate, but tend to produce spectra bluer than observed.