  Our present knowledge of the properties of blazars mostly comes from small
samples of bright objects, especially regarding studies on their cosmological
evolution. Statistically well defined and completely identified samples of
faint blazars are very difficult to obtain. We present a new X-ray selected
sample of 62 blazars and blazar candidates reaching deep X-ray fluxes. We
relied on the availability of large catalogs of astronomical objects combined
with on-line services offering simple access to finding charts and magnitude
estimates. We built the sample cross-matching X-ray sources in the Swift
Serendipitous Survey in deep XRT GRB Fields catalog with data from deep radio
and optical surveys. Our sample can probe populations of sources 10 times
weaker in the X-ray flux with respect previous studies, thus allowing for a
more detailed comparison between data and simulated counts. We use the sample
to calculate the radio and X-ray LogN-LogS of blazars down to fluxes at least
one order of magnitude fainter than previous studies. We show that, considering
that our sample may be somewhat contaminated by sources other than blazars, we
are in agreement with previous observational and theoretical estimations.
