1. Root rot perhaps? When the roots get too much moisture, they can develop this fungal infection. Its symptoms are almost identical to that of lack of water (wilting) because the roots are not able to supply the leaves with the necessary moisture. Additional watering, in this case, does not cure the problem.

You can try to water only when soil feels almost dry. What is difficult in this case is that there is no way to tell if the plants will make it.

2. Some forms of anthracnose, another fungal infection, can cause the leaves to darken. It starts with dark brown to black circles that spread on the leaf. The leaf looks wilted and eventually falls off. The blotches can spread to the blooms as well.

Here is a link that might be useful: Hydrangea Diseases