The ideal time to cut hydrangea blossoms for drying is toward the end of the season, August through October, when the larger petals are starting to fade or change color and the tiny flowers on top of the colorful petals are just beginning to open. If you can't really see the tiny flowers on your hydrangea variety, going by the changing shades of color is just fine.
Don’t worry too much about being exact. Hydrangeas are very forgiving flowers. In fact, you can simply let them dry on the plant, until the petals feel papery. You might not get the best color and they won’t last as long as the water dried method below, but it sure is easy to do. The only time drying hydrangeas on the plant is a bad idea is during a rainy season. The flowers will turn brown before you get a chance to dry them.