Apple and white grape are my two go to bland slightly fruity aromatics juices to fill volume. I rarely add water to a ferment. ,,,
OK the blueberry should dominate with apple hiding in the background, if you are lucky you might get an apple with pH 3.7 to 4.0 which helps push the pH up a bit. TA wise apple can be OK and the blueberry I have access to is high. Some folks do MLF on apple which would help cut the acid.