Blueberry Pie Filling: Can you answer a cooking conundrum?

At closing time today, one vendor at the Boston Haymarket had blueberries on sale at $5 for 2 4.5 pound crates. We live in the North End, and my wife always heads over there near closing time on Saturdays, because whatever they haven’t already sold will be marked down to incredible discounts if it’s perishable. So we got 9 pounds of blueberries for $5. That’s an incredible $0.56 a pound for beautiful fresh blueberries.

We can eat a bit fresh. But to save the rest, I’m cooking up enough blueberry pie filling for 3 pies, and freezing most of the rest. My blueberry pie filling recipe called for 3 tbsps. of cornstarch or 6 tbsps. of flour per each 4 cups of blueberries. I used cornstarch, but I think I was a tad short on having enough for 12 cups of blueberries.

My question
If the filling isn’t firm enough to stay in place in a slice of pie when it cools completely, can I thicken it to the Cinderella point by adding a bit of flour at a time and stiring it in cold, or must it be up to cooking temperature for the flour to fully interact with the juices?