to figure out the sugar content of the puree the easiest way to get in the ballpark is to do a mixture of the puree with water and then take a SG reading, then calculate based on how much you diluted it.
With blueberries, I would just start with a pound per gallon and see what you think. You might even go to 2 lbs per gallon or more before you get the flavor you want, it depends on what you like.