I don't know, but I'm thinking that in some parts of Saxon England it was customary to blow a horn at certain times and places to announce changes - sort of early town crier thing. I can't find a reference, but I seem to recall something about horns being blown at boundary stones in Yorkshire. Spelling was incredibly dodgy right up to modern times and in the early medieval period, blue could well have been spelled "blew". Then again - the Blew Stone may well have been one or the boundary stones where you blew your horn.