The darkening might be due to the growth of some sooty mold, which is a fungus harmless to the tree itself which grows on honeydew residues. As explained in the linked page, honeydew is a sugary liquid excreted by sap-feeding insects, which can include aphids, leafhoppers, Spotted Lanternfly, and soft scale insects. It's hard to tell which of those this tree might be hosting, since we can't see them in the photos (as they'd be up in the canopy). You could have a certified arborist or licensed tree expert inspect the tree to look for indications of a pest outbreak or any other issues that might be causing canopy thinning and decline. While arborists can't cure every ailment (and not all of those insects above would need to be treated or responsible for dieback), they can help to narrow-down a diagnosis, plus look for sources of tree stress that might be able to be alleviated.

Miri