My 2 cents here - red grape concentrate will ruin blueberry wine by giving it that Concord/foxy smell and taste. Use sugar.
If you are worried about losing volume to bench trials, don't. I bench trial in very small volumes. 37.5ml samples, in which 1/8tsp gran sugar is equal to 50g/gallon. As @Scooter68 said, undershoot a little bit, because it can taste sweeter after it has aged.
Another thing I've started doing is sweetening up a couple of bottles, chilling, and sampling again. It requires extra carboys but it really helps ensure you get the level of backsweetening just the way you want it. It requires extra carboys - take your 6-gal and rack it down to a 5, 1, and two 1/2 gallon carboys. Then stabilize, backsweeten and bottle one of the 1/2 gallon carboys. You'll get two bottles, try one after it has chilled and try the second one in 30 days.