Preheat your oven to 400°F (200°C). Prepare a standard 12-cup muffin tin by lightly greasing the top surface of each cup, or use silicone muffin top molds if available. Do not use paper liners.
In a large bowl, whisk together the gluten-free flour blend, granulated sugar, baking powder, and salt. Add the lemon zest and mix well to release the oils.
In a separate medium bowl, whisk together the melted butter, milk, egg, vanilla extract, and lemon juice until combined.
Pour the wet ingredients into the dry ingredients. Mix gently with a spatula until just combined. Do not overmix; a few lumps are acceptable.
Gently fold in the blueberries.
Spoon the batter onto the prepared muffin tin tops, filling each cup nearly to the top edge to encourage a large dome. If using, sprinkle the tops with coarse sugar.
Bake for 18 to 22 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Let the muffin tops cool in the pan for 5 minutes before carefully transferring them to a wire rack to cool completely.
Notes
For extra crisp edges, chill the batter for 30 minutes before baking.
If using frozen blueberries, do not thaw them; toss them lightly in one teaspoon of the dry flour mixture before folding them into the batter.
This recipe focuses on the top shape; the batter quantity is calibrated for a large dome rather than a full muffin.