  The X-ray spectrum of extreme HBL type blazars is located in the synchrotron
branch of the broadband spectral energy distribution (SED), at energies below
the peak. A joint fit of the extrapolated X-ray spectra together with a host
galaxy template allows characterizing the synchrotron branch in the SED. The
X-ray spectrum is usually characterized either with a pure or a curved
power-law model. In the latter case, however, it is hard to distinguish an
intrinsic curvature from excess absorption. In this paper, we focus on five
well-observed blazars: 1ES 0229+200, PKS 0548-322, RX J1136+6737, 1ES 1741+196,
1ES 2344+514. We constrain the infrared-to-X-ray emission of these five blazars
using a model that is characterized by the host galaxy, spectral curvature,
absorption, and ultraviolet excess to separate these spectral features. In the
case of four sources: 1ES 0229+200, PKS 0548-322, 1ES 1741+196, 1ES 2344+514
the spectral fit with the atomic neutral hydrogen from the Leiden Argentina
Bonn Survey result in a significant UV excess present in the broadband spectral
energy distribution. Such excess can be interpreted as an additional component,
for example, a blue bump. However, in order to describe spectra of these
blazars without such excess, additional absorption to the atomic neutral
hydrogen from the Leiden Argentina Bonn Survey is needed.
