The following powerpoint was graciously shared with me by Max Edgley, a PhD candidate at the Tasmanian Institute of Agriculture. I think this is a very useful exposition of a common problem in blackberries and I am very grateful for him sharing it with all of us.

The information below is taken from Max's presentation.  Max wrote in to stress that we please be aware that this information is from one season across and is as such very preliminary.

Red druplet disorder, or reversion, is the post-harvest reddening of blackberry fruit. While we understand that there is a loss of anthocyanin pigment in the affected druplets, it is still unclear as to what exactly is the cause of this disorder. 

Physical damage to the druplets has been implicated, as have rapid changes in temperature – ie from the hot of the field to the sudden cold of the cooler which seems to swell and then shrink the cells walls of the fruit. Fruit which is harvested at temperatures with an internal temperature above 22 C (that's 72.5 F) before cooling tends to show symptoms the most. 

In short, the work here investigates the above implications by testing staged cooling of freshly harvested fruit, physical damage, and the effects of different levels of fertilizer nitrogen in mitigating red druplet disorder.

http://www.utas.edu.au/__data/assets/pdf_file/0009/856422/Red-Drupelet-disorder-update-June-2016.pdf

Again, many thanks to Max for sharing this document with all of us.