James---there is no "right" or "wrong." But when adding acid blend to, what can be a very acidic fruit, your PH goes even lower. And acid blend is adding back some malic that you're trying to reduce by using 71B culture.
I don't know what the beginning PH is on the blackberries you are using. But ours are almost all wild and because out soil here is so acidic, the beginning PH is 2.8 or 2.9 on them. So it requires calcium carbonate to bring the PH up to 3.4 which is an acceptable PH on blackberry.
Some people have more of a tolerance for tartness--that's why I say there is no "right" or "wrong." But when many people on this forum are working with this fruit for the first time, it's wise that they follow good practices or else they end up with such an acidic wine that they won't be able to fix in the post ferment, and they'll be very frustrated with the advice they've been given.
Technically, you add acid if your PH is too high from your desired target PH, and calcium carbonate if your PH is low. All fruits have a dominate acid to them---and I'm unfamiliar with your desire to balance the 3 acids. Can you elaborate?