Certified GKH Gardening Expert
If the leaves that are turning black are near the bottom of the rosebushes, it could just be that they are dieing as part of the life cycle of the rosebush. The lower leaves sometimes become shaded by the upper leaves as the bush grows and are not as productive to the bush as they once were. These leaves will at times turn yellow then dark colored and fall off unless removed at the yellow stage. Another thing that could be making them turn black is a strong attack of the Downy Mildew fungus. Usually Black Spot is just some black spots on the foliage and not something that makes the entire leaf black, unless it is an extremely advanced stage of black spot. Both of these fungal bad boys love the cool moist spring time conditions.
Before actually trying to treat what the disease might be, take a sample of the leaves in a plastic sandwich bag to your local greenhouse or nursery. Ask for their staff Rosarian or if you have some certified consulting rosarians in your area try to contact them to come take a look at the bush or bushes with the black leaves. If I may be of assistance to you in locating a local Consulting Rosarian, please let me know. The American Rose Society maintains a listing of Consulting Rosarians in various States that stand ready to assist rose gardeners with just such issues.
If it turns out to be a severe case of black spot, a series of spray applications of a fungicide called Mancozeb will stop it in its tracks. I have personal experience with this product and it works very well. Should it turn out to be Downy Mildew, the products I know of to control it and prevent it are Heritage and Aliette W.D.G, given they are expensive but do work well. The attack will slow down as humidity decreases and good air movement around and through the bush is maintained, however it will not be controlled without some fungicide spraying. The leaves that are damaged by these funguses will not usually turn back to green once the funguses are killed and controlled. If only a few leaves are affected, removing them and spraying the bushes with a fungicide like Green Cure, Banner Maxx, Honor Guard or Immunox would be recommended to prevent further attacks.
Usually with Black Spot there are little black spots not an entire leaf turning black. It could be some form of leaf burn or scorch. This can happen due to a chemcial application that is too hot such as too strongly mixed insecticide or fungicide. It can also happen if a local dog has decided to pee on the rosebush, unfortunately I know this one well. If you see it happen and wash the foliage off right away there will be little impact. Otherwise there can be alot of damage such as many leaves turning black. A weed spray that has drifted onto the leaves can certainly cause damage that will cause some stages of leaf distortion and eventurally turning brown to black.