  Blazars are sources whose jet is pointing to us. Since their jets are
relativistic, the flux is greatly amplified in the direction of motion, making
blazars the most powerful persistent objects in the Universe. This is true at
all frequencies, but especially where their spectrum peaks. Although the
spectrum of moderate powerful sources peaks in the ~GeV range, extremely
powerful sources at high redshifts peak in the ~MeV band. This implies that the
hard X-ray band is the optimal one to find powerful blazars beyond a redshift
of ~4. First indications strongly suggest that powerful high-z blazars harbor
the most massive and active early black holes, exceeding a billion solar
masses. Since for each detected blazars there must exist hundreds of similar,
but misaligned, sources, the search for high-z blazars is becoming competitive
with the search of early massive black holes using radio-quiet quasars. Finding
how the two populations of black holes (one in jetted sources, the other in
radio-quiet objects) evolve in redshift will shed light on the growth of the
most massive black holes and possibly on the feedback between the central
engine and the rest of the host galaxy.
