your plants have sooty mold fungus which develops from the excretions of the pests that feed on the leaves.
not fatal, just unsightly.
to control the problem, get rid of the pests by applying a systemic insecticide, horticultural oil spray or insecticidal soap.

Here is a link that might be useful: sooty mold

I would like to suggest light pruning for your Gardenia to help it's speedy recovery. Remove any suckers and new cross shoots to keep the center open and airy to deter insects. You want to protect your eyes and nose. Also wear gloves and protective clothing. Keep your growing area clean. Pick up all fallen leaves and get rid of them. Hose down the plant thoroughly with water.

Another effective method of control is Homemade remedies from the kitchen in treating whitefly and sooty mold. You can do a web search to find the recipes.

Good Luck!

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THANK YOU BOTH SO MUCH FOR THE HELP, I HAVE PLANTED THE GARDENIA'S AROUND THE FENCES OF OUR PROPERTY FOR SEMI-PRIVACY AND I REALLY HATE TO LOOSE THEM. THANKS AGAIN FOR THE INFO!