A former president of the Houston Rose Society gave a talk at a local nursery and said that "Aliette" could be sprayed on roses once before any fungus appeared and would act as a preventive all season. I have not found it locally or tried it; I generally don't spray at all. I would be really interested to know if anyone has used it and with what success.
Last summer was dry - DROUGHT - but our climate is humid like yours usually. I plant mainly old garden roses and especially Earthkind roses that resist or at least survive blackspot. And I just resign myself to the fact that some of my roses will be leafless in the summer.
In our extremely SE Louisiana humid climate, we have to accept that there will be black spots when we decide to grow roses. One can start spraying all sorts of chemicals that may or may not prevent/cure black spots. However, be careful with all these chemical sprays especially if you have children or pets. If you ever pick up a rose to smell it before you put it in a vase, and you have sprayed it, then you also risk inhaling a possible chemical. I maybe a little too cautious, but I think one can still enjoy roses without resorting to use of chemicals.