This holly has an infestation of a sap-sucking pest insect - likely scale, given your description. The black crusty material is sooty mold, a fungus that does not infect the plant but rather grows on the sugary waste excreted by the insects. When the insects are controlled, the sooty mold will disappear on its own. Although it blocks some light from reaching the leaf surfaces, it doesn't cause any significant damage.

For the scale, you can treat the plant in June with horticultural oil to help smother the more vulnerable juvenile "crawler" stage, and a slightly more concentrated "dormant oil" to manage overwintering scale. The product labels will guide you as to dilution (unless they're ready-to-spray) and how often to re-apply. Thorough coverage of all plant surfaces, including undersides of the leaves, is important for effectiveness. Given the size of this tree, spraying may be difficult or impractical. Beneficial insects that prey on scale tend to arrive to control their numbers as well, so you can also forgo spraying if the scale populations aren't extensive or if you can live with the sooty mold for the time being.
https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs

Miri