It seems that almost every blueberry flower I look at has a slit on the side of it. The cause of this damage is Carpenter bees (Xylocopa sp.). A good, brief discussion on Carpenter bees (and other blueberry pollinators) can be found at this link: http://www.ent.uga.edu/bees/pollination/other-pollinating-bees.html#carpenter
Carpenter bees steal nectar from the flower with only slight pollination of the flower. Since they must create a hole to reach the flower (they are too large to go through the natural opening), they make a new shortcut for other insects as well, thus short-cutting the pollination process. Even though there is now a hole in the flower, other bees will likely continue to visit the flower — some may use the short cut or the natural opening.
When we had the freezing conditions a few days ago, I wondered if having the slit in the side predisposed the flowers to more damage. Hard to know. My guess is probably not, as the flowers that are damaged already are open and the cold air could contact the sensitive flower parts anyway. Below is a photo showing the slits on the side of blueberry flowers: