I Grow Hydrangeas and Have Always Had Great Success
The hydrangea bush I planted in spring has started turning black everywhere. The flowers also turned black and fell off. This was from a nursery and a 1 and 1/2 foot bush just beautiful. I don't know what else I can do for it. I have always had good luck with these bushes.
Certified GKH Gardening Expert
It sounds like you have sooty mold. This article should help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
This is a fungus caused by pests. I would treat the plant with neem oil. It is effective against the pests responsible as well as the sooty mold. Here is more information that may help you: