Spray a 12-count muffin pan with nonstick spray or use cupcake liners.
In a small bowl, whisk together the flours, baking powder, baking soda, salt, and cinnamon. In a large bowl, whisk together the mashed bananas, coconut oil, egg, and milk. Gently stir the dry ingredients into the wet ingredients and stir until combined. Fold in the blackberries.
Divide the batter equally between the muffin cups. Preheat the oven to 425°F and rest the batter while the oven heats.
Bake the muffins at 425°F for 5 minutes, then lower the temperature to 350°F and bake for an additional 20 minutes, until a toothpick inserted in the center comes out clean.
Remove from the oven and cool on a rack for 5 minutes, then gently turn the muffins onto a wire rack to cool completely.
Notes
Separate the blackberries: If using frozen blackberries, thaw the berries slightly, enough so that you can gently separate them from each other. You want to be able to fold in the berries individually so that they are evenly distributed throughout the batter—if they are frozen in a big clump this will be impossible!
And fold them in gently: This goes for frozen and fresh blackberries, but if using frozen blackberries be especially gentle when mixing them in so that they don't bleed into the batter.
Make it dairy-free: Substitute non-dairy milk for the whole milk.
Adjust the sweetness: Depending on how ripe your bananas are, you may wish to lower the amount of sugar by ¼ cup.
Use regular or gluten free flour: Substitute 1 ½ cups of all-purpose flour for the combo of oat and almond flour. Or use 1 ¾ cups homemade gluten free flour blend or your favorite gluten-free flour blend.
Serving options: Plain, warm, spread with butter, or cream cheese!