Lanternfly excrement. Apparently it's sweet because I would see yellowjackets following them closely and defending them by trying to keep me away as I closed in for a kill. Also, I've read that honeybees are attracted to it to and then produce dark or even black honey.

What I hate is the black rain. A large maple in my yard provides the only real shade but they loved that tree too and everything under it got stained black. Gross.