  There is mounting evidence that ultra-energetic neutrinos of astrophysical
origin may be associated with blazars. Here we investigate a unique sample of
47 blazars, $\sim 20$ of which could be new neutrino sources. In particular, we
focus on 17 objects of yet unknown redshift, for which we present optical
spectroscopy secured at the Gran Telescopio Canarias and the ESO Very Large
Telescope. We find all sources but one (a quasar) to be BL Lac objects. For
nine targets we are able to determine the redshift (0.09~$<$~z~$<$~1.6), while
for the others we set a lower limit on it, based on either the robust detection
of intervening absorption systems or on an estimation derived from the absence
of spectral signatures of the host galaxy. In some spectra we detect forbidden
and semi-forbidden emission lines with luminosities in the range $10^{40} -
10^{41}$ erg s$^{-1}$. We also report on the spectroscopy of seven blazars
possibly associated with energetic neutrinos that partially meet the criteria
of our sample and are discussed in the Appendix. These results represent the
starting point of our investigation into the real nature of these objects and
their likelihood of being neutrino emitters.
