Buds are tight and still developing.
Chilling has not been met for most cultivars (500 hrs in Salisbury).
Low temperatures predicted in the next 10 days are nearing the level that we have some concern about damage.
Bud Development. We are heading into the coldest part of the year and the blackberry and raspberry buds are tight and are at the hardiest stage of their lifecycle. Research done by Takeda and others in 2003 (http://www.ars.usda.gov/SP2UserFiles/Place/19310000/FTakeda/2003JASHS-blackberrypaper.pdf) followed stages of bud development of both thorny and thornless blackberry cultivars. In general in January buds are at stages 4-6. At this stage, the flowers are still developing, and the most terminal flower buds are furthest along. See the table 1 below for a detailed description of the stages. So, I dont think that the buds have fully developed. Will there still be some damage? Possibly. I will be monitoring the situation. The good news is that we know that there are secondary buds that can produce nearly a full crop.
Chilling. We have accumulated 508 hours of chilling as of today at the Piedmont Research Station in Salisbury, NC. This is based on the model found at http://nc-climate.ncsu.edu/cronos/blackberry. The model we use accumulates 1 chill unit when temperature is between 0 and 7° C. The plants should accumulate several more units this week, despite the predicted cold weather. Most of the varieties we grow are higher chilling and have not yet met their standard chilling units, which are estimated to be between 700 and 1100 hours. http://teamrubus.blogspot.com/2012/02/chillin-in-nc-part-2.html
Low temperature damage? Michelle Warmund did some research several years ago and found that with eastern types of blackberries, buds at this time of year have reached their maximum hardiness and are killed at −9 °F. While the types grown in the western part of the country are often damaged when temperatures reach 12 °F. However, we have observed damage to plants at the Upper Mountain Research Station in Laurel Springs to eastern types when temperatures dipped into the single digits. However, I think most of the damage we saw occurred later in the spring when the flowers were more fully developed.
What to do?
Monitor your crop. Monitor the temperatures.
Walk through your fields now and look at buds. Are they tight? Slice a couple open longitudinally and see if they are white and green inside, this is a healthy bud. Black is not good.
Use row covers? There has been success in the midwest using row covers over the RCA trellis. The trellis is in the lowered position allows for "easier" positioning of the row covers over the trellis. I am not sure how feasible this would be with the trellis system we use in NC. However, if you have a piece, it may be worth pulling it over the entire canopy, sealing up sides and ends. What you want to do is to hold in some heat that is captured during the day. Here is a picture of one of our RCA plots with row covers.