I have black leaves on my gardenia bush and there is very little foliage.
Black Leaves on Gardenia Bush
Certified GKH Gardening Expert
It sounds like it might be sooty mold, which is ultimately caused by pests. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
You have a fungus. use fertilizer with antifungal in it, or spray and antifungal on it.