The black residue is a fungus, but fortunately not a disease; instead of infecting the plant, it is growing on secretions on top of the leaf surfaces from a pest insect. Sooty mold is the name for the black growth, and it will eventually wear-off on its own in time, though it may take awhile. The insects producing the secretions (called "honeydew") are likely scale, as they are fairly common pests on holly. Whether you decide to intervene with insecticide treatments (either applied yourself or via a professional pesticide applicator) or not depends on how high the population is and if any beneficial insects have started to reduce their numbers. If treatments are made, applications will need to be thorough and likely repeated at least once this growing season, depending on the active ingredient.

Here is more information on this type of scale:

https://extension.umd.edu/resource/soft-scales-trees-and-shrubs

https://extension.umd.edu/resource/cottony-camellia-scale-shrubs

Miri

You're welcome.

No, the fungus is not known to be harmful to wildlife.

If this worsens at some point in the future and you decide to try pruning in lieu of pesticides, hollies respond well to trimming if they are otherwise healthy. This shrub appears to be in decent shape, scale aside. Therefore it should regrow from trimming suitably well, though it may take more than a season to fill-in a gap depending on how much it needs to regrow. So long as light is reaching the stems in question, that is, since very shaded growth (especially if shaded by its own branches) may not respond with bud-break and would instead stay bare. Since a drastic cut-back as a measure to deal with an out-of-control future heavy infestation would expose all of the remaining stems to sunlight (assuming the plant is in a sunny exposure), sluggish regrowth and gaps in the canopy would be less of an issue in that case.

Either way, you can certainly let nature run its course for now.

Miri