"Garden Q&A" item in the New York Times earlier this summer. It was about what affects the colors of hydrangea blooms. I've always heard about blooms changing colors from red to blue, or the other way a round. the article referred to the availability of aluminum in the soil. I thought it was interesting, but not very relevant to me.
Fast forward a few weeks and I started noticing our hydrangea about to bloom again - and the blooms are pink. It's been a while, but I could have sworn they blue earlier in the year. We transplanted from the back yard just before it bloomed this spring. I found the first photo of the hydrangea when it was still in the back yard. It's definitely blue, but there is a hint of pink in there.
Now look at the plant - the flowers are definitely pink. I have heard about people changing the pH of the soil, just to change the color of the flowers, but I have never seen it before. Very interesting!