Preheat the oven to 400F. Line a 12 cup muffin tin with paper liners or grease with nonstick cooking spray. Set aside.
In a medium bowl, whisk the gluten-free flour blend, granulated sugar, baking powder, ground cinnamon, baking soda, and salt together. Set aside.
In a large mixing bowl, add together the milk, eggs, vegetable oil and vanilla extract. Whisk until smooth.
Pour the wet ingredients into the dry ingredients. Mix until combined.
Carefully fold in the blackberries, being sure not to overmix so that the batter does not get dyed with blackberry juices.
Portion out the muffin batter into the prepared muffin tin. The muffin cups will be almost filled to the top.
Bake at 400F for 15-17 minutes, until the muffins are browned and cooked through.
Let the muffins cool for 5 minutes before removing from the muffin tin. Set on a wire rack to finish cooling.
Combine the powdered sugar and lemon juice in a small bowl. Whisk until smooth. If too thin, add more powdered sugar. If too thick, add some additional lemon juice until you’ve reached your desired consistency.
Drizzle the glaze over the blackberry muffins.
Serve and enjoy!