Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large bowl, whisk together the all-purpose flour, 3/4 cup granulated sugar, baking powder, baking soda, and salt until well combined.
In a separate medium bowl, whisk together the melted butter, buttermilk, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix; the batter should be slightly lumpy.
Carefully fold the fresh blueberries into the batter, ensuring they are evenly distributed.
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.
While the muffins are baking, prepare the lemon meringue topping. In a clean mixing bowl, beat the egg whites with an electric mixer on medium speed until foamy.
Add the cream of tartar and continue beating until soft peaks form.
Gradually add 1/2 cup granulated sugar, one tablespoon at a time, while beating on high speed until stiff, glossy peaks form.
Gently fold in the lemon juice and lemon zest until just combined.
When muffins are done, remove them from the oven and reduce oven temperature to 350°F (175°C).
Spoon or pipe the lemon meringue topping generously over each muffin.
Return the muffins to the oven and bake for an additional 5 minutes, or until the meringue is lightly golden on top.
Remove from the oven and allow muffins to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.