Black marks on your hydrangea sound like a fungal infection. It's been wet or humid or you're getting water on the plant's leaves. First, get rid of any fallen leaves. Then make sure your plant has good airflow and isn't crowded. Only water at the soil level when the soil is dry 3 to 4 inches down (poke your finger in the soil to check). When it's dry, water deeply enough that you end up watering about twice a week. Also, make sure the soil drains well and the plant's "legs" aren't wet. If this continues, you may need a fungicide spray.
Black marks on your hydrangea sound like a fungal infection. It's been wet or humid or you're getting water on the plant's leaves. First, get rid of any fallen leaves. Then make sure your plant has good airflow and isn't crowded. Only water at the soil level when the soil is dry 3 to 4 inches down (poke your finger in the soil to check). When it's dry, water deeply enough that you end up watering about twice a week. Also, make sure the soil drains well and the plant's "legs" aren't wet. If this continues, you may need a fungicide spray.