As you may know, you should never rinse blueberries until you are ready to use them. Rinsing removes the "bloom," which is the waxy, silvery-white coating that appears on blueberries, grapes, and some plums. Bloom acts as a barrier against insects and bacteria, and helps to seal in the fruits' moisture. Rinsing breaks down this protection, ultimately allowing the berries to cook more efficiently. For the purposes of this recipe, rinsing also adds just enough liquid to the mixture to allow the tapioca to begin working its magic before the berries begin to burst during baking. Do not skip this step, or your filling may not cook correctly.