  Popular internal shock models, developed to explain production of high energy
flares in blazar jets, involve collisions between local overdensities of matter
being ejected by a central engine and moving along the jet with different
velocities. Prior to such collisions, the matter is relatively cold and
therefore does not produce intrinsic non-thermal radiation. However, due to
Comptonization of external radiation by cold electrons, the presence of such
matter should be apparent by prominent precursor soft X-ray flares, visible
prior to non-thermal $\gamma$-ray flares. In this paper we discuss the
predicted properties of such precursors and study the dependence of their
properties (luminosities and light curves) on kinematic parameters of
relativistic ejecta and on an angle of view. We demonstrate that the lack of
evidence for luminous soft X-ray precursors can be reconciled with our
predictions for their properties if acceleration and collimation of a jet takes
about three distance decades. We briefly discuss the severe constraints on the
internal shock models that would be imposed by a non-detection of such
precursors.
