It's not Black Spot, which is a common rose fungal leaf infection, but it is another fungal leaf spot (probably Cercospora). Leaf spots are very common on this type of hydrangea, and in years with wet spring weather (as we had this year), or in situations with recurring leaf wetness (like with irrigation that "rains" on the foliage), they can be more prevalent. Despite the eyesore, such leaf spots are fortunately not a serious threat to long-term plant health.

Fungicide will not cure the plant of existing disease, and while its use as a preventative (in spring, well before symptoms manifest) might suppress most infections, it will need recurring use and might cause harm to other organisms (pollinators, beneficial fungi, etc.) in the process. Given that expense and hassle, plus the fact that such approaches don't always work completely (rain could interrupt a spray schedule, allowing infection to take place), we generally do not recommend fungicide use.

You can rake-up the shrub's fallen leaves later this autumn and dispose of them, to reduce the risk of infection next year from spores overwintering in the debris. If you irrigate the plant during drought (always recommended to reduce its overall stress), either avoid wetting the leaves or water early enough in the day so that foliage can dry by nightfall, since prolonged wetness can make leaves easier for spores to infect.

Miri

Miri, thank you!

You're welcome!