Preheat oven to 400 degrees F (200 degrees C). Grease a 9×9-inch glass baking dish.
Pour peaches into the prepared dish, and sprinkle the blueberries over the peaches. In a bowl, whisk the flour, sugar, and baking powder until thoroughly combined. With a pastry cutter, cut the butter into the flour mixture to make a crumble, and spread the crumble mixture over the fruit.
Bake in the preheated oven until the fruit is bubbling and the topping is golden brown, 45 minutes to 1 hour. Allow to cool; filling will thicken as it cools.
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