Answer: Your zip code places you in zone 6A which is getting very cold for growing Hydrangea macrophylla successfully. Most varieties bloom on the old wood from the year before. If winter kills off the branches or if spring frosts damage the early buds, then the plant will not bloom. (And if you prune it in fall, winter, or spring or early summer, it will not bloom for the same reason.)
The Endless Summer variety however is able to bloom on both old growth and on new growth of the season. If the winter kills off the old growth, it can still bloom later in the summer and into the fall on the new growth. In climates where the plant does not die back at all in the winter, this can make its bloom season seem "endless". In climates such as yours where it is normally too cold in winter to bloom these hydrangeas, the late blooming on new growth allows the plant to bloom nonetheless.
To try to maximize the bloom potential of your hydrangeas, they should be grown in a spot that is a warm microclimate, sheltered and protected from the winter wind. You might consider a wind break if it is in the open. Hydrangeas need some direct morning sun or very bright dappled light all day in order to grow vigorously and set flower buds. The soil should be organic and humusy and evenly moist yet well drained like a wrung out sponge. You should mulch with several inches of organic mulch in a flat layer over the root area all summer, and add to that in late fall after a few freezes. (Do not heap it over the stems however.)
In the spring, trim away any winter killed wood but minimize your pruning as much as possible. If a late frost is predicted, cover the plant to protect it.
Ideally you would fertilize based on soil test results, but an annual top dressing of good quality compost along with some slow release 10-10-10 in granular form should be adequate. Read and follow the label instructions. To some extent the color is affected by pH. In your area it should be naturally acidic enough to bring out the blue color. It is better to wait until your plant has been in the ground for a few years and see what color it settles into. Some plants will shift a bit until thoroughly rooted in the native soil. Then if you want to adjust it you can do so, but be sure to do it slowly to avoid shocking the plant. Usually it is better to enhance the natural pH than to try to fight it.
The perennial Hibiscus can sometimes be started from root cuttings in early spring before it starts to grow, or can be rooted from tip cuttings taken in the summer. The tip cuttings are less stressful on the parent plant, so that is what I would suggest.
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