I had this from a reader the other day;
Q: What do I do about my gardenia leaves that are turning black and what do I do if I see little white bugs on them?

A: The black leaves sounds like “sooty mold” which is a type of fungus common to gardenias and citrus plants. It is actually the secretion of small bugs that create this mold which isn’t harmful to the plant – it only makes it look horrible. The remedy for dealing with the mold is the eradication of the bugs that suck the sap from your plants.
These are usually aphids, scale, mealybugs or a plethora of other small insects which can be removed with any type of horticultural oil or by introducing other predator insects into the area (such as ladybugs).
The black can then be cleaned off with soapy water.