Char, the restaurant we go to in Lewisville has the same thing, but it looks that way all the time. I'd be interested to know what causes it too, or whether it is the type, or what.

Well I just looked up "black bark on crepe myrtle" and got one answer that described it as azalea bark scale caused from aphids. Look it up on Google and they'll tell you what works on it Don't have it on my Crepes, am keeping my fingers crossed!