Well, my blueberry cider is very much like Apfelwein with blueberries, but with different sugar additives. There are those that argue that you should put them in for primary fermentation and those that argue secondary. My solution? both. I put 4 lbs of blueberries in primary and will rack onto another 3 or 4 lbs of blueberries when I rack to secondary.
One thing you do need to deal with is volume loss, once the blueberries are removed, they tend to take a lot of liquid with them through absorption. So try to over shoot your final volume with initial must. I have about 6 full gallons right now and only expect to get about 5 gallons when I am finally ready to bottle.