  The analysis of the recent WMAP source catalog shows that the vast majority
of bright foreground extragalactic sources detected in CMB maps are Blazars. In
this paper we calculate the contamination of CMB anisotropy maps by this type
of flat-spectrum, strongly variable and polarized extragalactic radio sources
using up-to-date results from recent deep multi-frequency surveys. We found
that more than 50 known Blazars (or Blazar candidates) are included in the
90/150 GHz BOOMERANG anisotropy maps, a factor > 15 larger than previously
reported. Using a recent derivation of the Blazar radio LogN-LogS we calculate
that these sources induce an average sky brightness of 0.2 Jy/deg^2,
corresponding to an average temperature of ~3-5 muK. Moreover, we find that the
associated level of fluctuations is of the order of C_{l, Blazar}= 1.3 10^{-2}
mu K^2 sr at 41 GHz. Taking into account both Blazar variability and the many
steep-spectrum radio sources that flatten at high frequencies, as well as the
contribution of radio-galaxies, we find that the level of residual fluctuation
due to discrete extragalactic foreground sources could be factor of ~2 - 3
higher than the above estimate. We show that the Blazar induced fluctuations
contaminate the CMB spectrum at the level of ~ 20-50 % at l = 500 and 50-100 %
at l = 800. Careful cleaning for Blazar contamination of high sensitivity/high
resolution CMB maps is therefore necessary before firm conclusions about weak
features, like secondary high-l peaks of the CMB power spectrum or very weak
signals like CMB polarization measurements, can be achieved.
