It may be that aphids are feeding on your plant, excreting a sticky substance called honeydew, which is weathering and turning into a black coating called sooty mold. In itself, it won't harm your gardenia. It should wash off easily (or weather away on its own), but it does indicate insect feeding, which can eventually compromise the health of your plant. I'd wash it off with a strong stream of water (which will also dislodge the insect pests). Do this regularly to discourage the pests and to help wash away the sooty mold. Best wishes with your garden!
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