  Blazars are a remarkable type of Active Galactic Nuclei (AGN) that are
playing an important and rapidly growing role in today's multi-frequency and
multi-messenger astrophysics. In the past several years, blazars have been
discovered in relatively large numbers in radio, microwave, X-ray and gamma-ray
surveys, and more recently have been associated to high-energy astrophysical
neutrinos and possibly to ultra-high energy cosmic rays. Blazars are expected
to dominate the high-energy extragalactic sky that will soon be surveyed by the
new generation of very-high-energy gamma-ray observatories such as CTA. In
parallel to the discovery of many blazars at all frequencies, the technological
evolution, together with the increasing adoption of open data policies is
causing and exponential growth of astronomical data that is freely available
through the network of Virtual Observatories (VO) and the web in general,
providing an unprecedented potential for multi-wavelength and multi-messenger
data analysis. We present VOU-Blazars, a tool developed within the OU,
initiative that has been designed to facilitate the discovery of blazars and
build their spectral energy distributions (SED) using multi-wavelength
photometric and spectral data that are available from public archives, or that
have been generated as part of the OU, initiative and are accessible through VO
services. VOU-Blazars implements a heuristic approach based on the well known
SED that differentiate blazars from other astronomical sources. The VOU-Blazars
outputs are flux tables, bibliographic references, sky plots and SEDs.
VOU-Blazars has been extensively tested during the selection of new high-energy
peaked (HSP) blazars recently published in the 3HSP catalog, and has been used
to search for blazar counterparts of Fermi 3FHL, Fermi 4FGL, AGILE sources, and
of IceCube astrophysical neutrinos.
