Place apples in a large bowl and toss with 3/4 cup sugar, 2 tablespoons cinnamon, nutmeg, and lemon juice. Cover the bowl and place in refrigerator.
Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
Beat softened butter and 3/4 cup white sugar together in a large bowl until creamy; beat in egg.
Mix 2 cups flour, baking powder, and salt together in a small bowl. Stir flour mixture into butter mixture, alternating with milk, until fully incorporated. Fold in blueberries.
Arrange apples in a single layer in the bottom of the prepared baking dish.
Pour blueberry batter over apples.
Mix 2/3 cup sugar, 1/2 cup flour, and 1 teaspoon cinnamon together in a bowl; cut in cold butter until mixture is crumbly.
Sprinkle crumb topping over batter.
Bake in the preheated oven until a toothpick inserted in the center of the buckle comes out clean, 40 to 45 minutes. Cool on a wire rack.
