Preheat the oven to 425 degrees. Coat a muffin tin with coconut oil cooking spray.
In a large bowl, combine flour, baking powder, salt, and cinnamon. Mix well.
In another bowl, whisk together eggs, white sugar, and brown sugar until combined. Add in milk, oil, and vanilla extract and whisk until well combined. Add the flour mixture to the wet ingredients and mix everything together until just combined. Don't overmix. The batter will be extremely thick. Very gently add most of the blackberries to the batter.
Spoon batter into prepared muffin tins, filling all the way to the top. Add the extra blackberries evenly to the tops of each muffin making sure to push them down gently. Sprinkle with coarse sugar. Bake at 425 degrees for 5 minutes. Keeping the muffins in the oven and the door closed, reduce oven temperature to 375 degrees and continue to bake for 13-15 minutes, or until a tester inserted into the center of the muffins comes out clean. Remove from the oven and allow to cool for 10 minutes. Serve and enjoy.