Preheat the oven to 425 degrees. Coat a muffin tin with coconut oil cooking spray.
Combine flour, baking powder, salt, and cinnamon in a bowl.
Whisk the eggs, white sugar, and brown sugar in a large bowl until combined. Add milk, oil, and vanilla extract and whisk until well combined.
Gradually add the flour mixture to the wet ingredients and mix everything until just combined. Don't overmix. The batter will be extremely thick.
Gently fold in mostof the blackberries into the batter.
Spoon batter into prepared muffin tins.
Add the remaining blackberries evenly to the tops of each muffin, gently pushing them down. Sprinkle the tops 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 baking 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 on a wire cooling rack for 10 minutes. Serve and enjoy.