Preheat oven to 375°F. Coat a 9-by-5-inch loaf pan with cooking spray.
Whisk buttermilk, brown sugar, oil and eggs in a large bowl. Stir in mashed bananas.
Whisk whole-wheat pastry flour, all-purpose flour, baking powder, cinnamon, baking soda, salt and nutmeg in a medium bowl.
Fold the dry ingredients into the wet ingredients and stir until just combined. Fold in blueberries. Transfer the batter to the prepared pan.
Bake until the top is golden brown and a wooden skewer inserted in the center comes out clean, 50 to 60 minutes. Cool in the pan for 10 minutes, then turn out onto a wire rack. Let cool for about 2 hours before slicing.