Suspect it is Cercospora leaf spot - a very common affliction of several hydrangea species. Typically it is considered more cosmetic than harmful but severe infections and resulting leaf drop could weaken the plant.

If this is the first season you've had this problem, I doubt you need to do anything. This is usually a rainfall/high humidity/warm temperatures problem. You can remove the worst looking foliage if you like but otherwise it is just a matter of cultural issues - avoid overhead watering and clean up well around the plants. Mulching can help to keep any lingering spores from splashing up.