If it was my beer, and I was adding blueberry juice, I'd do it about a month before bottling and let it ferment out.
Then, I'd see if it needed some lactose for sweetening once the sugars in the juice fermented out. If so, I'd add some lactose and still bottle with the correct amount of priming sugar (about 4 ounces by weight per 5 gallons).
Diluting with juice and calculating the priming sugars in it may work. Or it may blow up in your face. Literally.