Black spot is a fungal disease and you can get purpose designed sprays for it. Called Rose Clear in UK but you will have an equivalent over there. Spray fortnightly and make sure to pick up and burn any fallen infected leaves as the infection can remain in the soil over the winter.

If you live in one of the dryer parts of the U.S.--like my area--you can avoid it by not watering from overhead, or by sprinkling only in the early morning. Elsewhere, look for Streptomyces lydicus, a fungus that eats other fungi.