That black stuff is sooty mold growing on honeydew a substance secreted by some insects. It could be aphids, scale, whiteflies or mealybugs. I suspect scale or whiteflies because it is a gardenia. These insects will be found underneath the foliage.
I suggest you take a sample of the leaves with insects to your County Extension Office. They can assist you with diagnosing the specific cause of the problem and by recommending an appropriate, labeled spray product. It will most likely be a horticultural oil or perhas a systemic insecticide. Your local Home Depot garden center will then be able to direct you to the recommended product.
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