  The spatially resolved broad-band spectroscopy with Very Long Baseline
Interferometry (VLBI) is one of the few methods that can probe the physical
conditions inside blazar jets. We report on measurements of the magnetic field
strength in parsec-scale radio structures of selected bright Fermi blazars,
based on fitting the synchrotron spectrum to VLBA images made at seven
frequencies in a 4.6 -- 43.2 GHz range. Upper limits of B <= 10^-2 -- 10^2 G
(observer's frame) could be placed on the magnetic field strength in 13
sources. Hard radio spectra (-0.5 <= a <= +0.1, S_nu ~ nu^a) observed above the
synchrotron peak may either be an indication of a hard energy spectrum of the
emitting electron population or result from a significant inhomogeneity of the
emitting region.
