Prepare pie crust. Pierce with fork and pre-bake empty crust according to package (or recipe) directions.
Whisk to combine sugar, cornstarch, water and salt.
In skillet or sauce pan, place 2 cups of fresh blueberries and add whisked sugar mixture to pan.
Allow mixture to thicken stirring/whisking frequently over medium heat. The mixture will become dark in color over several minutes.
Turn off / remove from heat, and add 1 Tbsp. butter. To lessen the pies sweetness, add up to 1 Tbsp. fresh lemon juice and stir both butter and juice to combine.
Stir in remaining 2 cups of fresh blueberries.
Place blueberry mixture into baked pie crust.
Refrigerate for one hour+ prior to serving.