Dixie...I would post this in the pests and problems area if I were you. I'm surprised what you've tried didn't work. The black coating is probably a mold that is feeding on "honeydew" secreted by aphids. You might try blasting them with a hard spray of water every day or 2 for a couple of weeks and see if that helps any. As for the scale, insecticidal soap or dormant oil might help...that should help with the aphids, too. Neem is a very good natural pesticide that you could try. I'm no expert on gardenias, though, so maybe someone else will be able to help.

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Dave
Even my growlights are getting restless!