Combine the milk and oats in a bowl and let the oats soak for 20 minutes.
Preheat oven to 425°F. Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. Set aside.
In a large bowl, combine the flour, baking powder, baking soda, cinnamon and salt.
Whisk together the melted butter, maple syrup, egg and vanilla extract until combined. Add the wet ingredients and soaked oat mixture to the dry ingredients, stirring just until combined. Gently fold in the blackberries.
Spoon batter into prepared muffin pan, filling them all the way to the top. Sprinkle some oats and coarse sugar on top if desired.
Bake for 5 minutes at 425°F and then reduce heat to 350°F. Continue to bake for 15-18 minutes or until a toothpick inserted in the center comes out clean. Allow muffins to cool for 5 minutes in pan before transferring to a wire rack to cool.
Notes
All refrigerated items should be at room temperature, which helps the batter mix easily and evenly.
If the blackberries aren't sweet, add an additional ⅓ cup of maple syrup.
Use an ice cream scoop to get an even amount of batter in each muffin tin.
You'll know the muffins are done when a toothpick comes out clean.