Bring the blueberries, apple, sugar, citrus juice, zest and cinnamon up to a simmer over medium heat, stirring occasionally, cooking until the blueberries are soft and tender, about 10 minutes. Whisk the cornstarch with a few tablespoons of cold water and add this to the blueberry filling, stirring and returning to a simmer until the filling has thickened and is glossy. Set aside to cool temperature. If making ahead, store the filling chilled, but bring it up to room temperature before baking into a pie.