This is likely a disease called anthracnose. Anthracnose pathogens have a broad host range, which includes a wide variety of commonly grown woody shrubs and trees. The fungus Colletotrichum can attack both leaves and blooms in the lower and upper part of bigleaf hydrangea plant canopy. Development of this disease is favorable in wet weather conditions. Heavily fertilized hydrangea may become more susceptible to anthracnose disease. First symptoms of this disease are the circular or slightly irregular brown spots forming on the leaves. The center of these brown spots turns to tan in color. Sometimes larger spots appear on border mid vein or other major veins in the leaf and they become clearly more angular in shape. When anthracnose developed under favorable environmental conditions, large, dark brown, irregular spots can spread across the whole leaves and flower petals. Several continuous days of wet conditions can produce masses of spores, which can be embedded in hydrangea leaf debris. Spores may spread by splashing water to the leaves and bloom clusters. Repeated showers, dew, and lengthened periods of heavy fog can speed up the rate of infection and the appearance of symptoms. The main source of anthracnose inoculum is the infected hydrangea. The following options are available for managing anthracnose on hydrangea such as taking cuttings from healthy plants; collecting fallen diseased leaves and removing blighted blooms from landscape plantings. Proper fungicide treatments can also protect susceptible hydrangea from anthracnose.

Here are some publications to check out:

https://www.uaex.edu/publications/pdf/FSA-7570.pdf

https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/general-care-hydrangeas

Hope this helps!