You have scale, and the black on the leaves is the honey dew that is left behind as the insect moves throughout the plant.
Scale insects suck sap from plants, robbing them of essential nutrients. The armored and soft scale bug is the most destructive. Armored scales are more difficult to control once mature (which is why Sussex Tree offers a spring preventative). Soft scale bugs excrete large amounts of honeydew, which enables the growth of sooty mold, a black-colored fungus that can stunt plant growth by blocking the process of photosynthesis. Mealybugs are easier to control with the proper program in place. Scales cannot fly, therefore, dispersal depends on the movement of crawlers. This year during winter scouting an abundance of all types of scale have been noticed due to the warm and dry fall/winter we have experienced. A systemic treatment should be done in the spring (End of March beginning of April) to treat as a preventative before warm weather brings the scales out of hibernation.