I suppose it is possible that the black spot fungus could become resistant after years and years of heavy use. I have not seen that yet though. One thing to keep in mind here. Even after the black spot fungus has been killed, the marks it leaves on the foliage will not disappear. Those marks are there for the life of each leaf. However the new foliage coming forth should be free of the black spot if the fungus has been killed. The mancozeb leaves a yellowish powdery residue on the foliage and that is part of how it works. Trying to wash that off or lessening it can effect how well it works. In some cases folks will mix the mancozeb and some Immunox together in the same spray tank for spraying on the roses as this does lessen the yellowish powdery residue it leaves. This can work well but may hinder how well the mancozeb will work in severe cases.
