The black residue appears to be sooty mold, a relatively harmless fungus that grows on sugary substances produced by pest insects. (The fungus does not infect plants and does not need treatment.) It can take awhile to weather off of plants whose pests have since disappeared.

Two insect pests of crapemyrtle can lead to sooty mold growth:  the aptly-named crapemyrtle aphid and crapemyrtle bark scale. The former tends not to cause serious harm to the tree and are more of a nuisance. The latter might cause poor blooming or branch decline if their numbers become high. The aphids are not yet out -- crapemyrtle foliage, which they feed on, won't appear until around May -- but you might have scale present higher in the canopy. You can use the images and descriptions on our new Crapemyrtle Bark Scale web page to learn what to look for, plus how to treat them if present.

If you don't seen any pests currently, just keep an eye on the bark and foliage as we move into spring and summer. Feel free to submit additional pictures if you see something then that you're not sure of.

Miri