Answer: I don't think that fireblight, a bacterial infection, could have taken hold and shown the characteristic blackening symptom in as short a time as one week, let alone overnight. Fireblight can, indeed, be spread by bees, but it can also spread by wind, and by pruning tools. To prevent infection by the latter, make a disinfecting solution of one part bleach to 9 parts water, and dip pruning tools between cuts. Some sources suggest using a pruning sealer over new cuts, though others think this mayslow down healing. The only thing I can think of that might explain the blackening is that the new cuts may have exuded some sap; then some type of sooty mold grew on the sap residue. If it is sooty mold, it will not harm the tree. To be on the safe side, I would prune off remaining sooty areas, carefully disinfecting pruners between cuts, and keep a very close eye on all your trees for signs of infection.
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