In my intro. to GR class we recently covered the Kruskal Szekeres diagram and trajectories within the diagram. My question comes from a comment made by my professor about time-like trajectories emerging from v=0 and that if an emitter sends a light signal to an observer it will be blue shifted. He said that this is a classical case of instability in the Kruskal-Szekeres geometry; however, I am unsure as to why this is (i.e. why is a blue shift not physical).