Hydrangeas respond to the pH of the soil where they are planted, so don't be surprised if the pink hydrangea you received on Mother's Day and lovingly planted in your garden changes color and blooms blue the following year. Your hydrangea is simply responding to the soil pH. Blue flowers are produced in acid soil (pH 5.5 and lower), and pink flowers are produced in alkaline soil (pH 7 and higher).
You can add aluminum sulfate around your hydrangeas to acidify the soil. Add lime to make your soil more alkaline. Some selections are less affected by soil pH than others. White-flowering ones, such as ‘Lanarth White’ and ‘Madame Emile Mouillere,’ will stay white.
We must have highly acidic soil here at the lake since what I've seen around is the bluest of blue. Yesterday I snuck out back and cut off some from my neighbor's back yard. He's a bachelor who totally ignores his plants and has the most beautiful blooms in the street. He won't even notice!
Top photo: Southern Living
Bottom photo: Lindaraxa