Question
My gardenia bushes has some Black substance on some the leaves I live in Florida

Answer
This sounds like sooty mold. Sooty mold fungi grows on a substance called honeydew. Honeydew is secreted by sucking insects like scales and aphids. These insect suck the plant juices from the plant and secrete a sticky substance which ifs high in sugar content. The fungi grows on the sugar. Control the sucking insects and the sooty mold will go away.
Treat the plant with an insecticide called Bayer Advanced Tree and Shrub Insect Control. It is applied to the soil around the plant and the roots transport the insecticide to the twigs and leaves and when the insect suck the juices the insect is killed. Check with your local nursery for this product. Here is a web link to more information on it.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html