Preheat oven to 350 degrees.In a large mixing bowl, cream sugar and butter until light and fluffy.Add eggs and vanilla or lemon extract, beat well.Add the flour alternately with the buttermilk; beat until smooth.Pour mixture into the unbaked pie shell and bake for 45 to 50 minutes. (Note: Cover pie with foil if top is browning too quickly.)Meanwhile, prepare sauce by combining sugar, cornstarch and water in a small saucepan. Cook over medium-high heat, stirring occasionally, for 5 minutes or until thickened. Stir in blueberries; simmer for 10 minutes or until blueberries burst. Add butter, stirring until melted.Serve pie either warm or cold, topped with fresh blueberry sauce.