I've had Black-eyed Susan's along the side of my house for years; and about three years ago. I noticed the leaves turning black. They didn't get as tall and the flowers didn't get as big. The leaves get black spots, then the entire leaf gets covered, dries up and dies. There are way few flowers now. Used to get lots of flowers. Are they just old? Do they have a blight or a bug?
Black Eyed Susan’s
Certified GKH Gardening Expert
It sounds like your black-eyed Susan's have a mildew or fungus. Make sure you clean the bed out thoroughly this fall and dispose of all the material by burning. In the meantime, treat your plants with an all purpose fungicide, or neem oil. As our gardens grow, changes take place that we never notice. Do your plants get the same air circulation they used to, the same sun? Maybe it's time to find a new home for your black-eyed Susan's and a new plant for the side of your house.