There is also a thing with the acid levels in blackberry. Did your source press the berries or steam them. Sometimes the acid levels are so high, a dose of potassium carbonate can help drop the pH down some. You could make an ok blackberry 3 gal of pure juice in 5 but 100% juice is more impressive though harder to balance, this is one reason why you almost always see blackberry wine as being made very very sweet. It also takes oak very well, and if you switched to making a melomel you could add 5 quarts of honey at the beginning and bring to vol up to 5 gal and make a very good melomel. WVMJ