  We present a new method for identifying blazar candidates by examining the
locus, i.e. the region occupied by the Fermi gamma-ray blazars in the
three-dimensional color space defined by the WISE infrared colors. This method
is a refinement of our previous approach that made use of the two-dimensional
projection of the distribution of WISE gamma-ray emitting blazars (the Strip)
in the three WISE color-color planes (Massaro et al. 2012a). In this paper, we
define the three-dimensional locus by means of a Principal Component (PCs)
analysis of the colors distribution of a large sample of blazars composed by
all the ROMA-BZCAT sources with counterparts in the WISE All-Sky Catalog and
associated to gamma-ray source in the second Fermi LAT catalog (the WISE Fermi
Blazars sample, WFB). Our new procedure yields a total completeness of c~81%
and total efficiency of e~97%. We also obtain local estimates of the efficiency
and completeness as functions of the WISE colors and galactic coordinates of
the candidate blazars. The catalog of all WISE candidate blazars associated to
the WFB sample is also presented, complemented by archival multi-frequency
information for the alternative associations. Finally, we apply the new
association procedure to all gamma-ray blazars in the 2FGL and provide a
catalog containing all the gamma-ray candidates blazars selected according to
our procedure.
