During the summer when fresh berries are plentiful, replace 2 cups of the uncooked blueberries with 2 cups of raspberries.
Whisk together the cornstarch and water in a large saucepan until smooth. Add 3 cups of the blueberries and the sugar. Bring to a boil over medium-high heat, stirring frequently and pressing the blueberries against the side of the saucepan with a wooden spoon to crush them. Cook, stirring constantly, until the mixture thickens and boils, about 1 minute.
Remove the saucepan from the heat and stir in the remaining blueberries. Pour the filling into the prepared crust. Refrigerate until the filling is set, at least 5 hours or up to overnight. Cut into wedges. Yields 1/8 pie per serving.