Preheat oven to 375°F (190°C) and line a 12-count muffin tin with paper liners.
In a large mixing bowl, sift together the gluten-free flour blend, granulated sugar, baking powder, and salt.
In a separate medium bowl, whisk the milk, melted butter, egg, and vanilla extract until a homogenous emulsion is formed.
Create a well in the dry ingredients and pour in the liquid mixture; stir with a spatula until just combined, ensuring no dry flour pockets remain.
Gently fold the fresh blackberries into the batter using a rubber spatula to avoid crushing the fruit.
Distribute the batter evenly among the 12 muffin cups, filling each to approximately 3/4 capacity.
Bake for 20 to 25 minutes, or until the internal temperature reaches 200°F (93°C) and a tester inserted into the center emerges clean.
Cool the muffins in the tin for 5 minutes before transferring to a wire rack for final cooling.