|Blackberry bud damage in April 2007. Note that the primary buds were killed (browned leaves) and secondary buds growing (green leaves).|
Although we don't have data on critical temperatures for blackberry, we expect that damage due to cold would be very similar to strawberry. Tighter buds are less vulnerable, and open blossoms are the most vulnerable. These temperatures are:
Tight Bud: 22F
Popcorn stage: 26F
Open Blossom: 30F
Strawberry growers can use rowcovers or irrigation to protect open blooms. Blackberry growers do not really have these options. Rowcovers would be very difficult to spread over the rows. Overhead irrigation adds ice and therefore weight to the canes and will cause them to break.
We experienced a similar situation in 2007. There was lethal damage to primary buds. Fortunately, secondary buds broke soon after the primary buds were killed and the secondary buds produced approximately 70% of a full crop. Five to six weeks after the freeze, the secondary buds were open and you could hardly tell there was any damage earlier that year. Laterals were longer and harvest was later, I think it was at least 2 weeks. Given that we were early, this delay if it happens may not be a bad thing, we may be able to reset our harvest to the normal dates.
For some insight into what happened in 2007, here is a link and some pictures.
There is also more information on strawberry protection and critical temperatures on the strawberry portal.