Interesting case documented in the pictures below of PrimeArk 45 blackberry in which the fruit did not pollinate very well.  The issue is limited to one area of the field, where the grower suspects it wasn't quite moist enough during the hot spell of two weeks ago.

The problem does seem to be limited to a certain age of fruit in that area of the field, and if one recalls two weeks ago during this hot spell we didn't have much customary cooling at night. 

This makes sense, since as readers know, high temperatures reduce the amount of viable pollen and consequently the success rate of germination on the pistil.  It is good to know as a field diagnostician that the peripheral pistils on the flower become receptive first, and as a rule not all pistils are receptive at the same time.  This goes some length to explaining the unevenness of pollination and subsequent lack of druplet formation.