Question: what product(s) have you been using to prevent disease? Please be very specific, brand names, product names, etc. Then we can advise.

It takes a while for the blackspot spores to begin to show on the leaves. The chances are your plants were initially infected three weeks or so before you started seeing the evidence. At that point using a typical preventative spray won't help much. You need to attack the disease spores.

One of the few fungicides that can have an effect on the spores is Manzate (Mancozeb). You should spray your roses with Manzate 3 times, 3 days apart. Avoid doing this in the heat of the day or it may burn the foliage. Once you have done this, then begin spraying with a fungicide made to prevent new blackspot attacks, following the instructions on the label as to amount and timing.

I add Manzate to my spray mix fairly often since I do see a leaf or two with blackspot from time to time and I'm not about to let it really take hold. Anyone else have a different approach?