This depends on what you're looking for. Some people are trying to make a Cream Ale with a 'hint' of blueberry. Detectable, but not overpowering. This is the style I would be looking to make. Others, however, have no problems making a 'Blueberry Drink' with a hint of beer. This can be attributed to the over-use of either flavoring route - fruit or extract. Typically, fruit extracts in beers, in my experience, are detectable as extracts - although I've had a good Apricot Ale that used extract. I would use about 2 lbs of blueberries, preferably fresh - but frozen would work. With extract, 1.5 oz would be a nice hint of blueberry. You don't want to overpower the 'Cream Ale' aspect of the beer, but let both elements stand out.