Not sure of the lineage of the Blackberry, but in my past endeavors, blueberry plants have always been light feeders that reacted badly to overfeeding. Only had 1 blueberry that tolerated nutes well, BlueDigiBerry, and she was pure blueberry muffin. But she was a photoperiod. I may have to grow her again while hitting her with some auto pollen. Ihave so many plans to get to and I am sure I won't get to most of them. I do see the slight yellowing on the newer leaves, just be careful upping the nutes with her. Good Luck, and good growing.