Hello, Olympiad. From your description of the bloom color (blue), it would seem that the hydrangeas are mopheads. These types of hydrangeas can live in very cold zones but their blooms (and-or stems) do not survive winter. This type of problem is sometimes called planting out of zone. As a result, when growth begins in the Spring, all grow originates from the crown or base & last year's stems look dried out and lifeless. If the mopheads only blooms once a year, you get no flowers.

Mopheads normally develop flower buds in July-August. To make sure that the stems & their flower buds survive your winter, you will need to apply winter protection techniques. Make sure you do not prune the mopheads in July or later because then you will be getting rid of the Spring blooms.

Reblooming mopheads.... there are mopheads for sale that develop flower buds in July-August, bloom in Spring and then rebloom all over again. They do this multiple times throughout the growing season. Reblooming mophead hydrangeas can be of help in very cold locations like yours. If the winter cold and winds dry out the stems or kill the flower buds, these plants will leaf out from the crown and develop new flower buds so you will get blooms no matter what. The Endless Summer Series and the Forever & Ever Series are some examples of these.

Too much fertilizer is also a possibility. Hydrangeas do not need fertilizer like roses do and a single application will last for the whole growing season. However, if you use a fertilizer nigh in nitrogen multiple times through the year then at some point, the plant will be producing nice green leaves and little to no blooms. A soil pH kit sold at plant nurseries can help determine if your soil's nitrogen levels are high.

Too much shade or dense shade can also contribute to bloomage problems. However, based on your comments, these shrubs are getting enough sunlight.

Luis