Sooty mold is a result of aphids dropping honeydew on the leaves, which then provides the medium for sooty mold growth. Keep an eye on the underside of the leaves next spring/summer and spray with insecticidal soap to get rid of the aphids. Its of no use to spray now.

Are you sure it wasn't mildew? Crepes are notorious for mildew, especially in hot, humid days of late summer. You can thin the limbs by trimming from the center outward to develop an open space inside to allow air to circulate better and get more sunlight. Space the bushes so they are not crowded. Replace them with mildew resistant varieties. A lot of the newer varieties are mildew resitant.

Black stuff on a crape myrtle is certainly black sooty mold. The mildew that these plants are prone to is powdery mildew, a white or gray disease of the leaves. Pterostyrax is correct that the black sooty mold is caused, most probably, by aphids (could be scale). However, I disagree that it is of no use to spray with dormant oil. Aphids overwinter as eggs in the nooks and crannies of our plants. Spraying with dormant oil is one of the BEST methods of suppressing a major outbreak come spring. I would spray three times during the winter and again in the spring just as the buds start to swell. Dormant oil applications are a very important part of IPM for Crape myrtles.

In the spring and summer keep your eyes out for aphids on the new growth and be prepared with the insecticidal soaps.

I think that pruning will help you as much as anything. Pruning also makes the Crepe Myrtle a lovely addition in the winter. Make sure that you get all of the lower branches and provide amost a "top heavy" look for these trees. You will love the look! With the added circulation of air, I think you will not have a problem. This took care of my problem.

My crepe mytrle is turning white flowers at top with salmon color at bottom! What is it doing? Is it mold starting ? Last year it was all salmon color!