  The infrared properties of blazars can be studied from the statistical point
of view with the help of sky surveys, like that provided by the Wide-field
Infrared Survey Explorer (WISE) and the Two Micron All Sky Survey (2MASS).
However, these sources are known for their strong and unpredictable
variability, which can be monitored for a handful of objects only. In this
paper we consider the 28 blazars (14 BL Lac objects and 14 flat-spectrum radio
quasars, FSRQs) that are regularly monitored by the GLAST-AGILE Support Program
(GASP) of the Whole Earth Blazar Telescope (WEBT) since 2007. They show a
variety of infrared colours, redshifts, and infrared-optical spectral energy
distributions (SEDs), and thus represent an interesting mini-sample of bright
blazars that can be investigated in more detail. We present near-IR light
curves and colours obtained by the GASP from 2007 to 2013, and discuss the
infrared-optical SEDs. These are analysed with the aim of understanding the
interplay among different emission components. BL Lac SEDs are accounted for by
synchrotron emission plus an important contribution from the host galaxy in the
closest objects, and dust signatures in 3C 66A and Mkn 421. FSRQ SEDs require
synchrotron emission with the addition of a quasar-like contribution, which
includes radiation from a generally bright accretion disc, broad line region,
and a relatively weak dust torus.
