I brewed a blueberry Kolsch that won numerous 1st places in the Fruit Beer Category last year. The key is to retain enough of the base beer's qualities so that the fruit complements the beer without overly dominating it. However, you want to have fruit flavor and aroma. My blueberry kolsch used 7.5 lbs of blueberries for a 5-gallon batch and I tasted it daily until I was pleased with how it tasted. It only took a few days for the right flavor and aroma to be reached. I'd recommend freezing the berries and adding them to secondary.