Place the blueberries, sugar and lemon juice in a medium sauce pan and cook over medium-high heat, stirring frequently, until the berries are softened and release the juice, about 5 minutes.
In a small bowl, mix the water and cornstarch until blended. Pour into the berries and stir.
Add the zest and continue to cook the berries until thickened, about 5 minutes.
Add the butter, stir.
Serve at once over ice cream, cake, pancakes, waffles, cornbread or lemon-meringue pie.