First, determine if the black you're seeing is damage to the leaf itself, or if it looks more like a sooty coating.

If it's damage to the leaf itself, then it could be from physical injury, herbicide, or cold or heat damage (such as if the plant was near a flame).

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If it's a sooty coating that you can clean off with a wet rag, what you're seeing is sooty mold. The mold itself doesn't harm the plant, but it does mean your gardenia is infested with an insect such as aphids, mealybugs, or scales. What happens is these insects secrete sugar-rich droppings, and the mold grows on these droppings. Control these insects with an insecticide (either an organic or synthetic type) and then wash the leaves off with water to get rid of the sooty mold.