Ive brewed blueberry ale a few times and it doesn't matter how much blueberry I put in the fermenter, the beer doesn't have enough blueberry taste. So, I've settled on this process:
I brew my base beer, an APA or EPA base, light on the hops.
I wait for fermentation to kick in. Take a 1-3 lbs of blueberries, put them in a blender, add a couple shots of everclear (to suppress or kill any nasty fungi or bacteria that may be on the fruit), blend the stuff up, make sure its good and mixed by tasting several times, (you may want to get 4 lbs just in case.) Pour berry mixture into fermenter and watch the fireworks. I like to add the fruit during primary fermentation since the blended blueberries undoubtedly contain a lot of O2. I don't want to oxidize my beer by adding it to the secondary.
When the beer gets racked to the secondary, I will add 4oz of blueberry extract. This adds enough flavor to be noticed, but it isn't overpowering. The real blueberries add a nice purple tint and some aroma.