Instructions: 

1. Set Aside Some Blueberries: Reserve 1 pint of the blueberries for later use.​
2. Cook the Base Mixture: In a saucepan over medium-low heat, combine the remaining 3 pints of blueberries with the sugar, water, cornstarch, and cinnamon. Stir the mixture continuously until the sugar dissolves and the mixture thickens slightly, which should take about 10 to 15 minutes.​
3. Incorporate Reserved Blueberries: Once the mixture has thickened, remove the saucepan from the heat. Gently fold in the reserved 1 pint of blueberries to add texture and freshness to the filling.​
4. Add Butter for Richness: Stir in the cold butter pieces until they are fully melted and incorporated, giving the filling a smooth and rich consistency.​
5. Cool the Filling: Allow the blueberry pie filling to cool completely before using it in your pie crust or storing it for later use.

For more details and tips, you can visit the original recipe here