In a large saucepan, add the sugar, cornstarch, water, salt, and heat over medium, stirring until everything is well mixed. Add the 4 cups of blueberries, raise the heat and bring to a boil, stirring often until the liquid mixture has thickened to your liking.
Remove the pan from the heat, add the lemon juice and zest, stir then add the remaining blueberries, stirring gently.
To cool, add the filling to a large bowl and set aside. Use once cooled or refrigerate until needed.