The Crape Myrtle Aphid is causing the leaves to get a black coating. Crape Myrtle Aphids are small pale yellow insects that feed on plant sap. These insects secrete a sugary substance called honeydew as they feed. The black coating that you see is is called sooty mold. Sooty Mold forms on the honeydew secretion not directly on the leaves. To stop the problem you can kill the aphids with Insecticidal Soap, Ortho Systemic Insecticide, Hi- Yield Malathion or SunSpray Ultra-fine Oil.