Tall crape myrtles
What causes blackening of crape myrtle leaves (I have the tall variety)? The black does not wash off; the leaves become dry, leathery and then die off.
If they are turning black, it may be sooty mold, which is most often brought on by pests. Neem oil will take care of both the fungus and whatever pest is causing the issue. These articles will help you: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm