  The \textit{Herschel} Astrophysical Terahertz Large Area Survey (H-ATLAS),
that has covered about 642 sq. deg. in 5 bands from 100 to 500 $\mu\rm m$,
allows a blind flux-limited selection of blazars at sub-mm wavelengths.
However, blazars constitute a tiny fraction of H-ATLAS sources and therefore
identifying them is not a trivial task. Using the data on known blazars
detected by the H-ATLAS we have defined a locus for 500\,$\mu$m selected
blazars and exploited it to select blazar candidates in the H-ATLAS fields.
Candidates and known blazars in the H-ATLAS equatorial and South Galactic Pole
fields were followed up with the Australia Telescope Compact Array (ATCA) or
with the Karl G. Jansky Very Large Array (VLA), and matched with existing
radio- and mm-catalogues to reconstruct the spectral behaviour over at least 6
orders of magnitude in frequency. We identified a selection approach that,
combining the information in the sub-mm and radio domains, efficiently singles
out genuine blazars. In this way, we identified a sample of 39 blazars brighter
than $S_{500\mu\rm m} = 35\,$mJy in the H-ATLAS fields. Tests made
cross-matching the H-ATLAS catalogues with large catalogues of blazar
candidates indicate that the sample is complete. The derived counts are
compared with model predictions finding good consistency with the C2Ex model
and with estimates based on ALMA data.
