The "tons of rain" that you have had recently promoted the germination of fungal spores. Clip off the worst leaves and discard them in the trash. Consider using a copper-based fungicide.

Thanks, Luis! There are just a few leaves and so I’ll cut them off. I am wary of using too much fungicide as I had to spray all of my roses already this year - I’ll wait until the hydrangeas really need it and hopefully they won’t!