They are root hardy only. Bloomage may or may not be reliable (you have to try or see what hydrangeas blooms where you live).

The mopheads, ie, the ones which you say bloom "on old wood" actually have new varieties that can also bloom on old and new wood too. Old versions of mopheads used to bloom only on old wood (mostly) and now nurseries offer these new versions that bloom on both old and new wood.

You can use those that bloom on old and new wood to get more reliable bloomage. In warm places like mine, this means I see blooms in early spring and in the summer. If something happens that kills the stems, I get new growth and that blooms. In cold areas, the spring blooms may get zapped by winter but the new growth may or may not produce blooms.

The catch is... in cold zones, this works well only if the growing season is sufficiently long to let the mature new growth develop flower buds, open the flower buds and bloom. In some locations, winter shows up early and interrupts all this so you get no bloomage. Vendors that say that the plant is hardy mean that they are root hardy, not that they will necessarily bloom every growing season.

Both the old wood only and the old/new wood mopheads require that one prune dead wood by the end of May. That has not been much a problem for me. But if the hydrangeas that bloom on old and new wood do not bloom reliably in cold zones, you have to apply winter protection in the Fall and that is high maintenance... well, for some people. This is not an issue for me (yet); I do apply it down here & I can see it being a pain/problem eventually.

The solution is actually quite simple. As hyed has said he does, slowly start switching some of these blues to the 'new wood' only hydrangeas (the Annabelles and the paniculatas).

There are new versions of Annabelle that start blooming lime and pink colors. Sorry, no blues. These colors cannot be changed by amending the soil. But they can add some variety to a garden full of Annabelle white blooms.

If you really want to try the blue blooms, look to see if someone else has them in your neck of the woods, make sure the one you buy is a rebloomer and start small.