Preheat the oven to 425°F and grease or line a 12-count muffin tin with cupcake liners. If using a silicone muffin tin, grease with cooking spray.
In a large bowl, whisk together the oat flour, baking powder, cinnamon, salt and coconut sugar.
Use a spoon to make a well in the center of the dry ingredients. Add greek yogurt, oat milk, eggs, melted coconut oil and vanilla extract. Whisk until a batter forms.
Let the batter sit for 5 minutes to thicken.
Fold the halved blackberries into the batter, just until combined.
Scoop about ⅓ cup batter into each muffin liner.
Bake muffins for 5 minutes at 425°F, then lower the oven temperature to 350°F and bake for an additional 18-20 minutes or until a toothpick inserted into the center of the muffin comes out clean.
Allow muffins to cool for 15 minutes before removing from the pan and setting onto a wire cooling rack. Cool muffins completely before storing.