The dark residue is a fungal growth called sooty mold, and while it does not infect the plant itself, it can be a nuisance as it covers surfaces coated in honeydew, which is what it feeds on. Honeydew is the sugar-water waste produced by sap-feeding insects. In this case, we see a high population of scale insects on the bark. This relatively new pest in Maryland, Crapemyrtle Bark Scale, has spread very rapidly (given how frequently crapemyrtle are planted, this is not super surprising) in the past few years.

Scale insects can be difficult to eradicate. Information on the page linked above presents the options, ranging from gentle bark scrubbing to multiple insecticide treatments. While a few beneficial insects (ladybug larvae and others) can consume this scale, they will not get rid of them completely by themselves. An experienced and certified pesticide applicator might be helpful (or necessary, depending on the chemical involved) if you choose to treat the plant with insecticide, as they have good-quality spray equipment and will be able to determine when to make applications to minimize impacts on beneficial insects while maximizing impacts on the scale. Successful treatment might take well over a year to achieve significant scale population reduction.

When the scale are brought under control and their honeydew production wanes, the sooty mold will go away on its own. Weather will gradually help to remove its residues as well.

Miri