Blueberry-Maple Muffins

Preheat oven to 400°F. Coat 12 muffin cups with cooking spray. Grind flaxseeds in a spice mill (such as a clean coffee grinder) or dry blender. Transfer to a large bowl. Add whole-wheat flour, all-purpose flour, baking powder, cinnamon, baking soda and salt; whisk to blend. Whisk eggs and maple syrup in a medium bowl until smooth. Add buttermilk, oil, orange zest, orange juice and vanilla; whisk until blended. Make a well in the dry ingredients and stir in the wet ingredients with a rubber spatula just until moistened. Fold in blueberries. Scoop the batter into the prepared muffin cups. Sprinkle the tops with sugar. Bake the muffins until the tops are golden brown and spring back when touched lightly, 15 to 25 minutes. Let cool in the pan for 5 minutes. Loosen edges and turn muffins out onto a wire rack to cool slightly.