Answer: What you describe can either be a disease or an insect problem. If the black leaves are smooth on the surface, and the black is throughout the tissue, it sounds like a disease called Tar Spot. If the black is a coating on the surface of the leaves, something that you can rub or wash off, it's do to insect feeding.
Tar Spot disease is caused by the fungus Phacidium curtisii. Yellow spots appear on the leaves of American and English hollies in mid-spring. These turn reddish-brown and finally black by fall. In years of heavy rainfall berries as well as leaves are spotted.
Prevention and Treatment: Remove and destroy badly spotted leaves, prune to improve air circulation and overcrowding, and clean up and destroy fallen leaves.
Insects, such as aphids or scale can also cause a black coloration, but this time on the surface of leaves. It can be easily rubbed or rinsed off. It's called sooty mold and is the result of insect feeding then excreting a sticky substance called honeydew. The substance is just sweet enough to cause fungal spores to grow and turn the substance black. It will weather away after you've controlled the insect population. Check the undersides of the leaves and the stems for evidence of aphids or scales.
Best wishes with your holly!
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