Well, I smashed them and then I strained the mash with a metal strainer to collect only the juice and leave the skins behind. I racked my oatmeal stout to the secondary and just added the juice to it for a week. I actually bottled the 3 gallons of my oatmeal stout and racked the other 2 gallons to 1-gallon glass carboys with blueberry juice. I left it for about 7-10 days because the blueberry added some extra sugars to the beer. I looked at my old notes and I ended up using 3lbs of blueberries for 2 gallons (or 1.5lbs per gallon). The taste definitely came out, but I may suggest only using 1lb for gallon if you only wanted a light blueberry taste. For a wit, I'd probably stick to 1-1.25lbs per gallon.
From an old post/notes when I made it:
"It's been a while and I ended up using 3lbs of frozen blueberries and smashing them into a pulp (literally). Put the mashed blueberry pulp-juice into two 1-gallon secondaries evenly and racked on top of them."