  We use 3 years of data from the Swift/BAT survey to select a complete sample
of X-ray blazars above 15 keV. This sample comprises 26 Flat-Spectrum Radio
Quasars (FSRQs) and 12 BL Lac objects detected over a redshift range of
0.03<z<4.0. We use this sample to determine, for the first time in the 15--55
keV band, the evolution of blazars. We find that, contrary to the Seyfert-like
AGNs detected by BAT, the population of blazars shows strong positive
evolution. This evolution is comparable to the evolution of luminous optical
QSOs and luminous X-ray selected AGNs. We also find evidence for an
epoch-dependence of the evolution as determined previously for radio-quiet
AGNs. We interpret both these findings as a strong link between accretion and
jet activity. In our sample, the FSRQs evolve strongly, while our best-fit
shows that BL Lacs might not evolve at all. The blazar population accounts for
10-20 % (depending on the evolution of the BL Lacs) of the Cosmic X-ray
background (CXB) in the 15--55 keV band. We find that FSRQs can explain the
entire CXB emission for energies above 500 keV solving the mystery of the
generation of the MeV background. The evolution of luminous FSRQs shows a peak
in redshift ($z_c$=4.3$\pm0.5$) which is larger than the one observed in QSOs
and X-ray selected AGNs. We argue that FSRQs can be used as tracers of massive
elliptical galaxies in the early Universe.
