It is sooty mold, an organism that derives all its nourishment from insect or plant secretions. The most common substrate is honeydew, a liquid secretion produced by aphids, soft scales, mealy bugs, white flies, and some leafhoppers. Since sooty mold is present on your gardenia it means one or more of these insects are feeding on your plant and secreting honeydew. Both are problems for healthy plant growth.
Inspect the plant for insects, and take steps to control the insect pest. The form of sooty mold that develops on gardenia leaves resembles a layer of black tissue paper. To help remove it from the leaf, apply a solution of soapy water late in the afternoon and wash it off using a strong stream of water early the next morning. Repeat as necessary. You’ll remove 50 to 70 percent of the black covering, allowing leaves to utilize sunlight for photosynthesis. If nothing is done, the leaves will fall from the plant from lack of sunlight.
