Considering that the staging exhaust is both rapidly expanding and also rapidly moving away from the stage, I think the likelihood is far greater that any soot we see anywhere on it was deposited during the many seconds that the stage was enveloped in exhaust while going bottom first into the reentry slipstream. LOX concentration near the drain end of its tank certainly stands out in my mind as the evident agent for the clean line on the cold end of the tank giving way to increased adhesion at the warmer end of the tank. The seaside photos of the crane-supported stage seem to show the same soot distribution we've seen in other pictures. I have a little concern for the exposed components inside the interstage that are washed by the startup exhaust from the second stage engine, but no doubt these are robustly attached and insulated.