Popular internal shock models, developed to explain the 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. We discuss the predicted properties of such precursors.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html