  High redshift blazars are an important class of Active Galactic Nuclei (AGN)
that can provide an independent estimate of the supermassive black-hole mass
function in high redshift radio-loud AGN without the bias due to absorption
along the line-of-sight. Using the Cosmic Lens All Sky Survey (CLASS) we built
a complete radio flux-limited sample of high redshift (z>4) blazars suitable
for statistical studies. By combining dedicated optical observations and the
SDSS spectroscopic database, we obtained a sample of 26 blazar candidates with
a spectroscopic redshift above 4. On the basis of their radio spectrum we
distinguish between blazars and QSO with a Gigahertz Peaked Spectrum (GPS) like
spectrum. Out of the 18 confirmed blazars 14 constitute a completely
identified, flux-limited sample down to a magnitude of 21 (AB). Using this
complete sample we derive a space density of blazars with 4<z<5.5 of rho=0.13
(+0.05,-0.03) Gpc^-3. This is the first actual estimate of the blazar space
density in this range of redshift. This value is in good agreement with the
extrapolation of the luminosity function and cosmological evolution based on a
sample of flat-spectrum radio quasars selected at lower redshifts and it is
consistent with a cosmological evolution peaking at z$\sim$2 similar to
radio-quiet QSO. We do not confirm, instead, the presence of a peak at z~4 in
the space density evolution, recently suggested using an X-ray selected sample
of blazars. It is possible that this extreme peak of the evolution is present
only among the most luminous blazars.
