Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large mixing bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, light brown sugar, baking powder, baking soda, and salt until well combined.
In a separate medium bowl, whisk the melted butter, buttermilk, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together using a spatula until just combined. Do not overmix; the batter should be slightly lumpy.
Carefully fold in the fresh blueberries, distributing them evenly throughout the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake the muffins for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and set aside while preparing the lemon meringue.
Reduce the oven temperature to 350°F (175°C).
In a clean, dry mixing bowl, beat the egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form.
Gradually add 1/2 cup granulated sugar, about 1 tablespoon at a time, while continuing to beat on high speed until stiff, glossy peaks form.
Gently fold in the lemon zest and lemon juice into the meringue until evenly combined.
Top each muffin generously with the lemon meringue, swirling it decoratively with a spoon or spatula.
Return the muffins to the oven and bake for an additional 5-7 minutes, or until the meringue is set and lightly golden on top.
Remove the muffins from the oven and allow them to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.