It looks like a common form of hydrangea leaf spots, sometimes called cercospora. They are usually triggered by overhead watering or keeping the plant close to others (like they do at plant nurseries so they can water them all quickly).

Water the shrub by watering the soil -not the leaves- early in the mornings. Remove plant debris under the soil and replace the mulch if this developed in your garden. Allow for some separation between plants so the wind will dry out the wet hydrangea leaves and help minimize the growth of spores.

Cercospora is unslightly but it should not kill the plant. Since fallen leaves can help propagate the fungal spores, cut the leaves off and throw them in the trash when the plant goes dormant. Make sure you do not cut the stem but rather the little "stick" between the leaf and the stem (aka, petiole, leafstalk, etc).

Yearly outbreaks in late summer to early Fall should reoccur once the plant is infested but you can minimize them by using the clean sanitation practices mentioned above.

For effective control (not cure) of Cercospora leaf spot with a fungicide, begin applications when spotting of the leaves is first seen and continue applying that treatment as needed. Typically, protective fungicide treatments are suggested only on valued plants (either that cost a lot or that hold great emotional value) that suffer a lot of damage every year.

At the end of the growing season, most people elect not to apply fungicides (due to co$$$t). Fungicides with these active ingredients can be used when you have a MAJOR problem: azoxystrobin (Heritage; smallest application rate and largest repeat interval); chlorothalonil (Daconil); mancozeb (Dithane; Protect; othrers); myclobutanil (Immunox); thiophanate-methyl (Cleary's 3336).

I saw some oakleafs infested with rust several months ago. It is a similar problem so I decided not to buy those. Cercospora is not visible early in Spring so you usually do not know if the plants have it.

More info on the link below,
Luis

Here is a link that might be useful: ACES - Diseases Of Hydrangea

Thanks :) it was 7$ and its fairly large so I had to get it :)