The Black Dust is called sooty mold. sooty molds are a form of fungi that grow on the sugary honeydew secreted by Aphids, scales, the whitefly, and other insects that suck the sap from plants. sooty mold is an unsightly black powder that can quickly coat the leaves of a plant in just a few weeks. The fungus itself doesn't really harm the plant; it just ruins the plant's appearance. What you need to do is get rid of the insects that are secreting the honeydew that is attracting the sooty mold.