Thanks for writing. You can encourage hydrangeas to go blue by adding sulfur or aluminum sulfate to the soil; do this any time in spring, summer, or fall.
As far as how to prune them: The best time is right after they finish blooming. Shortly after the blooms fully fade, the plants begin making next year's flowers --- so pruning in fall, winter, or spring could result in cutting the blooms off before they develop. Learn more:
---Justin, Senior Garden Editor, BHG.com