I made a 1 gallon batch of black cherry apple cider. I used 1 gallon of apple juice with 32oz of 100% black cherry juice. I sampled it still the other day it taste amazing! I back sweetened with glycerin which smothed it out and gave it a bigger mouth feel. the ABV was 9.5 percent. Cant wait to try it carbonated.
I imagine if you use the same ratio with black berry juice it could be pretty good.
One thing to note though The black cherry juice even though I only added 32oz made my cider quit a bit more sour like citrus sour. Once the glycerin was added though the sour taste was gone and the true flavor really rang out. THis could be the same case with blackberry. It might make it more sour than regular cider.