  Blazars are the established sources of an intense and variable non-thermal
radiation extending from radio wavelengths up to HE and VHE gamma-rays.
Understanding the spectral evolution of blazars in selected frequency ranges,
as well as multi-frequency correlations in various types of blazar sources, is
of a primary importance for constraining the blazar physics. Here we present
the results of a long-term optical monitoring of a sample of 30 blazars of the
BL Lac type. We study the optical color-magnitude correlation patterns emerging
in the analyzed sample, and compare the optical properties of the targets with
the high-energy gamma-ray and high-frequency radio data. The optical
observations were carried out in R and B filters using ATOM telescope. Each
object was observed during at least 20 nights in the period 2007-2012. We find
significant global color-magnitude correlations in 40 % of the sample. The
sources which do not display any clear chromatism often do exhibit
bluer-when-brighter (bwb) behavior but only in isolated shorter time intervals.
We also discovered spectral state transitions at optical wavelengths in several
sources. Finally, we find that the radio, optical, and gamma-ray luminosities
of the sources obey almost linear correlations, which seem however induced, at
least partly, by the redshift dependance, and may be also affected by
non-simultaneousness of the analyzed dataset. We argue that the observed bwb
behavior is intrinsic to the jet emission regions, at least for some of the
analyzed blazars, rather than resulting from the contamination of the measured
flux by the starlight of host galaxies. We also conclude that the significance
of color-magnitude scalings does not correlate with the optical color, but
instead seems to depend on the source luminosity, in a sense that these are the
lowest-luminosity BL Lac objects which display the strongest correlations.
