Answer: If the leaves themselves are black, it indicates a fungal or bacterial disease. However, if the black substance is only a coating on the surface of the leaf and you can scrape it off with your fingernail or wipe it off with a wet paper towel, it is something called sooty mold. Sooty mold is the result of insect feeding - usually aphids or scales or mealybugs. When these pests feed they exude a sticky substance called honeydew. It has a high enough sugar content that it turns black - hence the name sooty mold. You'll have to carefully inspect your plant for signs of insects and spray to control them. Insecticidal soap works against these sucking pests but you have to carefully spray both upper and lower leaf surfaces and repeat sprays are usually necessary. If the plant is really overpopulated with pests you may want to simply replace it.
Hope this information provides some guidance. Good luck with your gardenia.
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