I had one razor that started to rust during honing (but it was not normal honing, there was edge repair going on), but that is the fastest I've ever seen. But that stuff came right off with metal polish. I have found that fast rust is not deep rust (it doesn't have time to move past the very surface), so it comes off really easily. Also, that was red, not black. Black near the edge is much more likely "lather burn" - the basic soap causes black tarnish. This happens if there is some leftover lather left on the razor (hence usually from the edge up) or if lather gets into and is not removed from the scales.

But if that did happen during a single shave (wasn't there before you started and was after), I think some coating on the blade (tuf glide or ren wax are the first two that come to my mind) would be imperative.