Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Grease a standard 12-cup muffin tin well, or line it with paper liners if you prefer easier removal.
In a large bowl, whisk together the gluten-free flour blend, granulated sugar, baking powder, and salt.
In a separate medium bowl, whisk together the milk, melted butter (or oil), egg, vanilla extract, lemon zest, 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 fine.
Gently fold in the blueberries.
Spoon the batter into the prepared muffin cups, filling each cup about three-quarters full. For best results in achieving a dome, use a large scoop and mound the batter slightly higher in the center of each cup.
If using, sprinkle the coarse sugar evenly over the top of the batter in each cup.
Bake for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean. The tops should be golden brown.
Let the muffin tops cool in the pan for 5 minutes before carefully transferring them to a wire rack to cool completely.
Notes
If you use frozen blueberries, do not thaw them before adding them to the batter.
For a dairy-free option, substitute the butter with melted coconut oil and use a non-dairy milk like almond or oat milk.
To get the best dome, ensure your baking powder is fresh and do not overfill the cups past three-quarters full; the high heat helps them rise quickly.