My Crepe Myrtle Has a Sticky Substance on Its Leaves
Leaves are sticky and branches have black coloration.
Certified GKH Gardening Expert
This is a sign of a pest infestation, either aphids or scale. The sticky substance is honeydew excreted from the bugs. Treat the tree with a pesticide. I prefer neem oil myself (https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm), but any insecticide will fix the issue.
The black substance sounds like a sooty mold issue, most often brought on by pests. Neem oil can also be used to get rid of the fungus too. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm