Without being able to see an actual leaf, I presume you have bacterial leaf spot on your hydrangea. This is treatable. You will need a fungicide with chloranthil as the active ingrediant. Your local true garden center & nursery should have the this type of fungicide. There are a few things you can do besides spray them with a fungicide.

1. When you water do not wet the foliage. Wetting the foliage will spread the bacteria. Always water at the based of the plant.
2. Do not keep the hydrangea soggy. Keep the area moist but not soggy.
3. Remove any fallen leaves — put them in the trash do not burn them.

Your plant should recover nicely once you have sprayed them.