The black substance is a parasitic fungus (sooty mold) caused by secretions (honeydew) from the Asian Wooly Hackberry Aphids, a small white insect, often mistakenly identified as whiteflies.
They do no permanent damage to the trees, but cause a mess under them. The honeydew also attracts hordes of small ants, that feed upon it.
Not much can be done at this point, but an application of systemic insecticide in the spring and again in late summer, can reduce the infestation next year, if they decide to return.
The recommended chemical treatment is imidacdoprid, sold as Bayer Advanced Tree & Shrub Insect Control.
There are also beneficial insects that eat the aphids, one is the Ladybird beetle.
Rb

yep, seen those little jokers before.

so, sounds like all of the trees are going to be infested forever, may as well just cut them all down or ? it's a nuisance, staining our sidewalks and driveway, staining all of our plants... these hackberry trees seem to do nothingbut make a mess anyway. wouldn't mind replacing them with some other kind of tree.