  We study the linear polarization of the radio cores of eight blazars
simultaneously at 22, 43, and 86 GHz with observations obtained by the Korean
VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the
frame of the Plasma-physics of Active Galactic Nuclei (PAGaN) project. We
investigate the Faraday rotation measure (RM) of the cores; the RM is expected
to increase with observing frequency if core positions depend on frequency due
to synchrotron self-absorption. We find a systematic increase of RMs at higher
observing frequencies in our targets. The RM--$\nu$ relations follow power-laws
with indices distributed around 2, indicating conically expanding outflows
serving as Faraday rotating media. Comparing our KVN data with contemporaneous
optical polarization data from the Steward Observatory for a few sources, we
find indication that the increase of RM with frequency saturates at frequencies
of a few hundreds GHz. This suggests that blazar cores are physical structures
rather than simple $\tau=1$ surfaces. A single region, e.g. a recollimation
shock, might dominate the jet emission downstream of the jet launching region.
We detect a sign change in the observed RMs of CTA 102 on a time scale of
$\approx$1 month, which might be related to new superluminal components
emerging from its core undergoing acceleration/deceleration and/or bending. We
see indication for quasars having higher core RMs than BL Lac objects, which
could be due to denser inflows/outflows in quasars.
