  We investigate the physical properties of the 10 blazars at redshift greater
than 2 detected in the 3-years all sky survey performed by the Burst Alert
Telescope (BAT) onboard the Swift satellite. We find that the jets of these
blazars are among the most powerful known. Furthermore, the mass of their
central black hole, inferred from the optical-UV bump, exceeds a few billions
of solar masses, with accretion luminosities being a large fraction of the
Eddington one. We compare their properties with those of the brightest blazars
of the 3-months survey performed by the Large Area Telescope (LAT) onboard the
Fermi satellite. We find that the BAT blazars have more powerful jets, more
luminous accretion disks and larger black hole masses than LAT blazars. These
findings can be simply understood on the basis of the blazar sequence, that
suggests that the most powerful blazars have a spectral energy distribution
with a high energy peak at MeV (or even sub-MeV) energies. This implies that
the most extreme blazars can be found more efficiently in hard X-rays, rather
than in the high energy gamma-ray band. We then discuss the implications of our
findings for future missions, such as the New Hard X-ray Mission (NHXM) and
especially the Energetic X-ray Imaging Survey Telescope (EXIST) mission which,
during its planned 2 years all sky survey, is expected to detect thousands of
blazars, with a few of them at z greater than 6.
