Follow the steps in order for the best result.
Begin by preheating your oven to 375°F (190°C). Prepare a muffin tin with liners or grease it well.
In a large mixing bowl, combine the whole wheat flour, all-purpose flour, baking soda, baking powder, salt, and cinnamon. Whisk together to ensure all ingredients are evenly distributed.
In a separate bowl, mash the bananas until smooth. Add the sugar, corn syrup, egg, and melted margarine (or oil). Beat until well combined.
Gently pour the banana mixture into the dry ingredients. Stir until just moistened. Be careful not to overmix; a few lumps are okay.
Fold in the blueberries, distributing them evenly throughout the batter.
Spoon the batter into the prepared muffin cups, filling each about 3/4 full (approximately 1/4 cup of batter per muffin).
In a small bowl, prepare the crumb topping. Combine the brown sugar, whole wheat flour, oats, and ground cinnamon. Cut in the margarine using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Sprinkle the crumb topping evenly over the muffins.
Bake for 18 to 20 minutes, or until a wooden skewer inserted into the center comes out clean. Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely. This recipe yields about 10 muffins.
Enjoy these delightful banana blueberry crumb muffins warm or at room temperature!