I sprayed my Crepe Myrtle for aphids, and although the aphids are all dead, my crepe myrtle's leaves are still black and droopy. I did this last year, and finally removed all the old leaves whereupon new growth began. Will that be necessary?
What’s Wrong With My Crepe Myrtle
The sooty mold that grows on the honeydew excreted by the aphids is what makes the leaves black and droopy. It's hard to get the mold off leaves unless you can handwipe each one. The most important thing is to spray the aphids as soon as they appear, before they have a chance to cover the tree with honeydew. This article tells more about sooty mold. https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm