There is some cold weather forecast for tonight and so a lot of uncertainty about frost control. What are the critical temperatures? What are the stages of development? How long will it stay cold? These are all factors that influence the amount of potential damage. Michigan State University has a very good visual of bud/flower stages and critical temperatures that can be found here:
To quickly summarize — if your plants are at early pink bud then 23-25 F are the critical temperatures of tolerance. If they are at late pink bud (corolla still closed) then the temps are 24-27 F. Once the flower is open then the temperature of concern is 28 F. After the fruit is set, then anything below 32 F could potentially cause damage. Rabbiteye blueberries may be even less tolerant of the cold, so they could possibly be hurt at temperatures of 30F or below.
The University of Florida has a thorough publication on how to protect blueberries from freezes. You can access it here:
If you are interested in the tolerance of other fruit then check out this link to an Auburn University publication: