Preheat oven to 425 degrees F. In a medium bowl, combine the dry ingredients - rolled oats (NOT quick cooking/instant/steel cut), flour, baking powder, baking soda, cinnamon, and ½ teaspoon salt. Stir to combine. In a separate large bowl, combine the sugar, apple sauce, egg, vegetable oil, milk, vanilla,. Mix until thoroughly combined and smooth. Add the flour mixture to the wet ingredients, stirring till just incorporated. Do not overmix.
Taste the blackberries, if sweet, no extra sugar is needed. If a bit tart, stir them with 1 ½ tablespoons of white sugar, then gently fold into the batter till just mixed.
Prepare a muffin pan with paper cups or grease the pan well. Fill the batter, almost to the top of the cups or pan. Sprinkle the tops with extra white sugar and rolled oats, if desired.
Bake at 425 degrees F for 5 minutes, then reduce the heat to 350 degrees F without removing the muffins. Bake another 15-17 minutes until cooked through. Allow the muffins to cool in the pan for about 5 minutes then remove to continue to cool on a wire rack. If you eat them while still hot, they'll be more crumbly!
Notes
My oven is recently calibrated and these took exactly 20 minutes to be done. It's better to go low on the cooking time. If cooked too much, these will be more crumbly.
Sub 1 egg with your favorite egg substitute or 2 tablespoons of apple sauce.
To make all gluten free, use your favorite 1:1 gluten free all purpose flour or oat flour.