I'm in zone 5 New York State. I'm going to cover my blueberry bushes tonight with a 32 gallon plastic trash can. its not worth it to take a chance. its 28 degrees already. I might see 18 for a low tonight.
The only buds you need to worry about are flower buds, if you are concerned with fruit production which do not happen for a few months. If you are talking about leaf buds you are fine, they are tough plants and they should be fine. Since blueberries have late season berries, the earlier the leave out the better. This coming from someone who has picked wild blueberries in Zone 2 for many years (actually some years falls into Zone 1---that's really cold in the winter)
Thanks everyone! Well, more than half the sheets popped off the plants anyway, so they were on their own after all. I will have to come up with a more effective method in the future.
Joanna, I don't know if you had the unseasonably warm weather like we did last week, but it's been so warm here that our plants are about 5 weeks ahead of schedule. The flower buds on my plants have really grown, but they have not actually flowered, if that makes sense. I wasn't sure how tender they would be. The TV weather guys kept making such a big deal about us having a "killing freeze" that I got nervous. It probably didn't even get below freezing here last night after all.