  Over the last decade more than five thousand gamma-ray sources were detected
by the Large Area Telescope (LAT) on board Fermi Gamma-ray Space Telescope.
Given the positional uncertainty of the telescope, nearly 30% of these sources
remain without an obvious counterpart in lower energies. This motivated the
release of new catalogs of gamma-ray counterpart candidates and several follow
up campaigns in the last decade. Recently, two new catalogs of blazar
candidates were released, they are the improved and expanded version of the
WISE Blazar-Like Radio-Loud Sources (WIBRaLS2) catalog and the Kernel Density
Estimation selected candidate BL Lacs (KDEBLLACS) catalog, both selecting
blazar-like sources based on their infrared colors from the Wide-field Infrared
Survey Explorer (WISE). In this work we characterized these two catalogs,
clarifying the true nature of their sources based on their optical spectra from
SDSS data release 15, thus testing how efficient they are in selecting true
blazars. We first selected all WIBRaLS2 and KDEBLLACS sources with available
optical spectra in the footprint of Sloan Digital Sky Survey data release 15.
Then we analyzed these spectra to verify the nature of each selected candidate
and see which fraction of the catalogs is composed by spectroscopically
confirmed blazars. Finally, we evaluated the impact of selection effects,
specially those related to optical colors of WIBRaLS2/KDEBLLACS sources and
their optical magnitude distributions. We found that at least ~ 30% of each
catalog is composed by confirmed blazars, with quasars being the major
contaminants in the case of WIBRaLS2 (~ 58%) and normal galaxies in the case of
KDEBLLACS (~ 38.2%). The spectral analysis also allowed us to identify the
nature of 11 blazar candidates of uncertain type (BCUs) from the Fermi-LAT 4th
Point Source Catalog (4FGL) and to find 25 new BL Lac objects.
