It's not, at least according to the handful of sources I checked. But if she means this past winter, 6b was probably more like 8a and with a little microclimate (brick porch?) effect, the plant felt 9a, it's supposed lowest hardy zone. A lot of the success of zone-pushing or oddities seems to be reliant on drainage, which would be at its' max in a pot on a porch, as far as locations go. This is a good example of how an experience with too much cold can put a plant that would never normally go dormant when kept as a house plant into dormancy. It's always worth giving those plants a 2nd chance to see if the entire life was snuffed out or just the foliage. Also, regarding zones and perennials in the ground, one can often have a plant survive, even for a few years in "the wrong zone" before it is truly frozen one year, but most plants in this unique condition never perform as well as they could in a more proper location. They struggle harder than they're supposed to. Zone guidelines are used for reasonable expectations based on years of experience, not unique or temporary occurrences.

For hydrangeas planted in the ground, you need to water them during winter if your ground does not freeze. Once a week or once every two weeks will do. Apply similar logic for container grown hydrangeas as well.

piperjane, I'm sorry to hear about your hydrangea but the good news is I believe everyone here is right! I purchased an annabelle this spring that was still dormant that was cut down very low to the ground and the employee at the shop said you can cut it down to the ground every year and still have blooms the following year. I double checked that when I got home and bookmarked the site for future reference. This is what it says about them: "Many people grow hedges of Annabelle and cut them within a few inches of the ground each fall so they will not be an eyesore during the winter. They will still bloom beautifully in the spring/summer, however this drastic pruning may not allow stems to increase in size, and they may need staking to hold up the large heads." I wish you luck but I think your annabelle will be a-okay :) Here is a link that might be useful: Hydrangea Site

I do not have it but unfortunately, I responded to a loooot of people who had blooming problems with Bombshell even after they waited a few years and did "all things correctly". I even went as far as contacting the wholesaler who blamed pruning even though many people had not pruned at all. Because they had already bought the shrubs, I suggested to wait 3 years max and replace it if it had not improved by then. You may want to do a search in the forum for more posts on Bombshell and also consider other similar paniculatas.