Fungicides only work preventively not curatively, so you need to start a spray program when the leaves are still healthy, before they start to show symptoms. Once BS is present, the best you can hope for is to begin spraying, and keep up the feeding and watering to encourage the rose to produce new growth that will be protected against disease by your spray program. If you can find it, Fertilome's Systemic Fungicide is a homowner friendly size version of BannerMaxx, one of the more effective fungicides to be introduced recently. Alternate that with Mancozeb, and your roses should be able keep their new foliage clean.
I live in the blackspot capital of the world - the north central piedmont of North Carolina. I have a preventive spray receipe that I have been using for 2 years that is flawless for controlling blackspot and insects (it came from my rose company - wear protective gloves and clothing when using). Mix 2 tablespoons of 50% Captan and 1 tablespoon of Halt systemic fungicide in a half cup of water. Put half gallon of water in sprayer and add mixture. Add one half tablespoon of slicker spreader and 3 tablespoons of Orthene systemic insecticide. Make up to one gallon. Mix and spray. Start spraying every 7 to 10 days in the spring as soon as you have 2 inches of new growth. Make fresh mix each time. Do not spray if the ambient air temperature is over 85 degrees Fahrenheit - it can burn the foliage. I have only had blackspot on 2 roses out of 100 - those 2 roses were purchased this spring and were infected when I bought them and they, of course, are being sprayed weekly with all the rest. I remove all contaminated foliage as it comes off the rose or pull it off. To have great roses, you must make a commitment to spray once a week. An hour a week is not too much to ask in repayment for the joy and satisfaction achieved from a beautiful bed of your favorite roses. And please forgive my longwindedness.