Iris photo of her hydrangea prompted me to post mine. To get that gorgeous deep blue color, the soil has to be just right -- acidic. Here is SE Ohio, our soil is clay -- great for pottery, not so good for planting until it's doctored up. I have to add aluminum sulfate to the soil around the bush every year to get any blue color to turn up at all. If I don't feed it, the color is white!
I was playing around with the flash at twilight, and shot this one:
I've added a link to a website I found that talks about the colors of hydrangeas and how to change the colors depending upon the pH of the soil.