Hi, 

This very typical for late season Hydrangea leaves and is a fungal leaf spot due to our weather conditions. It is not going to hurt the plant, despite being an eyesore. You can rake-up and throw out fallen hydrangea leaves if you want to try to minimize it, though it won't guarantee the infection doesn't return in a future year. 

You can read more about it on our Hydrangea pages: https://extension.umd.edu/resource/hydrangea-identify-and-manage-problems#:~:text=buds%20and%20leaves.-,Hydrangea%20diseases,-Bacterial%20leaf%20spot

Let us know if you have further questions. 

Emily