In many ways, blueberry pie filling is one of the quickest types to make because you don't have to peel, cut or prepare the fruit. But blueberries produce a lot of juice, so it's important to use the right thickener in the right amounts for a successful pie. In a pinch, you can use flour instead of cornstarch, but cornstarch produces better results.
Cornstarch makes the best pie, but you can achieve similar -- though not identical -- results with flour.
Why Use Flour?
The main advantage to using flour instead of cornstarch is its availability. All-purpose flour is inexpensive and sold in any grocery store. Even if you're out of cornstarch, you likely have some flour in your pantry. Flour can be combined with sugar and tossed with fruit, so it needs no special treatment.
The main drawback to using flour over cornstarch or other thickeners is its propensity to become cloudy or gummy. Blueberry filling made with cornstarch is typically translucent. Use quick-cooking tapioca or a specialty product formulated for thickening fruit pies, for excellent results. Flour can impart a starchy or flat flavor to blueberry pie filling. Blueberries tend to exude a lot of juice during cooking and flour doesn't have quite the thickening power of cornstarch. In general, flour works best for thickening heartier orchard fruit pies, such as apple or peach pies. Cornstarch is a better choice for delicate berry pies.
How to Use It
If you decide to use flour to thicken blueberry filling, plan on about 1 cup of flour for every 5 cups of fruit. If you're using frozen berries, increase the flour by 1/4 cup. Combine some of the sugar with the berries and let them sit for 30 minutes. Drain off any excess juice. Mix the remaining sugar with the flour and toss gently with the berries. You can also add lemon zest or cinnamon for extra flavor.
Try sprinkling some graham cracker crumbs in the bottom of the crust before you add the filling. The crumbs will absorb any excess juices so they don't make the crust soggy. Make sure you bake the pie long enough -- a golden-brown crust doesn't tell you if the filling is done and pies thickened with flour may thicken more as they cool. A blueberry pie is done when the filling has thickened slightly and is bubbling. Although it's tempting, wait to cut the pie for at least two hours. If you cut into a hot pie, the filling will ooze, making a soggy mess.