Blue hydrangeas are not pruned the same way most flowers are pruned. The reason is because blue hydrangeas produce flowers on old wood, rather than on new growth. This means that if you prune the old wood, as is normal for most plants, you will be removing the buds for next year's blooms. For this reason it is best to do the majority of pruning (especially for shaping purposes) late in the season, such as in August, after the plant is in full bloom.
Use your pruning shears to remove any obviously dead or diseased branches in early spring, before your blue hydrangea begins its yearly growth cycle. Make your cuts just above a leaf node. Do not trim any of last year's old wood unless you wish to shape the bush and are not concerned about sections that may not bloom later in the year.
Trim your blue hydrangeas for shape late in the year, after the blooms begin to fade. Trim only the growth that put on blooms this season whenever possible. Keep in mind that any of this year's growth that you remove may reduce the number of blooms for the following year, so prune sparingly. Always make cuts just above a node.
Top your blue hydrangea late in the growing season, once the blooms have begun to fade, if you wish to prevent the plant from getting taller. Again, remove only last year's growth (the growth with this year's blooms) whenever possible and trim as little of this year's growth as possible, since the current growth will produce blooms next season.