I don't like runny fruit pies, especially blueberry so when I make one, I thicken the fruit with tapioca, not flour or cornstarch which can cover up the flavor of the fruit. Some use arrowroot which is better than flour or cornstarch but if you really want the flavor to come out, use tapioca starch. If you prefer a really runny pie, don't use a thickener. Let the fruit mixed with sugar and tapioca sit for about 15 minutes filling the shell. For six cups of blueberries, you will need about 4 tablespoons of the tapioca but you can adjust that according to how runny or firm you want the pie to be.