Some crepe myrtle cultivars are naturally more resistant to aphids than others.
There is a lot of info online.
You can also try being preventative by spraying yours weekly with a neem oil emulsion. (Must be a freshly mixed batch each time you apply)
I had 4 Red Rooster crepes (black foliaged crepes) for a time and was diligent about spraying. They were gorgeous for a time but frankly, keeping black soot/aphids in check was a losing battle.
I also tried systemic preventative with weak results.
There is an online company I like, The Crepe Myrtle Company. You may find a cultivar you can love (instead of Black Diamond) there.
Good Luck!
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