Thanks for writing. I'm sorry, but without being able to see your crape myrtle, I can't say for sure why it's turning black. These plants are susceptible to a number of pest/disease issues. Your best bet is to contact your county extension office; a staffer there should be able to help more if you can describe the problem in more detail and answer their questions.
Sorry I can't help more, but this free service really isn't suited to diagnosing plant problems like that.