If you are using fresh blueberries, you'll need to add 1/2 cup juice to the mixture. If you can find blueberry juice in the store, that would obviously be great. If not, another all-fruit juice such as cherry or even grape juice can work. You can also use something like orange juice, and it will add a nice orange flavor to the blueberries which totally works. In a pinch, water can work too. Although the flavor won't be as good.
The butter is optional, but it adds a subtle richness to the filling.