Black spot is a fungal infection which particularly affects roses. Using Roseclear Ultra as often as it says on the bottle will help to control this condition - clear all infected, fallen leaves from soil, particularly at the end of the growing season, the fungus can overwinter in those and in soil. Alternatively, a more environmentally friendly treatment is cow's milk, mixed one part milk to two parts water in a sprayer, spray till run off. More info on this treatment here

http://www.agardenforthehouse.com/2012/05/got-black-spot-get-milk/