This is a pest called Crepemyrtle Aphid, and it specializes on this particular host plant. The black leaf coating is sooty mold, a fungus which grows on the aphid honeydew but which does not infect the plant itself. High levels of sooty mold do shade the leaves, however, which may be contributing to the lack of (or delayed) bloom in addition to the stress from the aphid feeding. The white flecks are shed aphid skins, which can be prominent when aphid populations are high.

Aphids are preyed upon by a wide range of insect predators and parasites, but if they are absent, then treatments may be useful. The simplest approach is to try to blast the aphids off the leaf undersides with a strong jet of water. Otherwise, insecticidal soap or horticultural oil can be applied to all leaf surfaces when temperatures are below 85 degrees. Repeat sprays of water or pesticide will be needed; follow product label instructions if using a pesticide. If the plants are large to too tall to spray, a professional pesticide applicator should be making the treatments instead.

https://entnemdept.ufl.edu/creatures/orn/trees/crapemyrtle_aphid.htm

Ted should also check the bark on his crepemyrtles for a relatively new pest, Crepemyrtle Bark Scale. Here's more information and images of what to look for (though this page is mainly written for horticulture professionals): https://extension.umd.edu/resource/crapemyrtle-bark-scale
If this scale is found as well, please let us know as we are collecting data on where it has been seen. (If unsure of ID, you/he can send us photos.)

Leafless upper stems may be unrelated - either winter damage, cicada damage, or an issue related to root health.

Miri

Thank you so much for this tremendous amount of good information to help Mr. Bradford!  

You're welcome!

Regarding the issue with the Crepe Myrtle aphids, can Mr. Bradford use Quadris with flint or pyrethrin?

That product appears to be a fungicide, and that is not recommended for treatment of the sooty mold; when the aphids are under control, the mold will go away on its own.

Pyrethrins are not recommended for aphid control; they're too broad-spectrum and potentially harmful to beneficial aphid predators. Horticultural oil or insecticidal soap are the preferred treatments (one of the two, not both at the same time) when water sprays won't be sufficient.

Miri