Without photos to illustrate the problem, it's hard to know exactly what the issue may be but with newly planted hydrangeas, I'd guess at Cercospora leaf spot. This is a very common fungal disease that seems to be most prevalent on nursery stock. It tends to be encouraged by conditions of high humidity - close placement of plants/lack of air circulation and overhead watering, both conditions that are typically present in a nursery setting.

This tends to be more of a cosmetic concern that a real health issue for the plants. The good news is is that if you follow good planting and cultural conditions, the plants tend to grow out of it in a season or two. So site and space carefully, avoid overhead watering and remove and destroy heavily affected foliage and clean up and destroy all the fallen leaves at the end of the season. And since most fungal spores have an ability to remain and live on the soil surface over winter, applying a good mulch around the plants will help to smother the spores and prevent them from reinfecting the plants each season.

Here is a link that might be useful: hydrangea diseases