Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups lightly.
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 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. Be careful not to overmix; some lumps are okay.
Fold in the fresh blueberries gently to avoid breaking them.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake the muffins for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
While the muffins bake, prepare the lemon meringue topping. In a clean mixing bowl, beat the 2 egg whites with cream of tartar on medium speed until soft peaks form.
Gradually add 1/2 cup granulated sugar, a tablespoon at a time, while continuing to beat on high speed until stiff, glossy peaks form.
Gently fold in the fresh lemon juice and lemon zest into the meringue.
Once the muffins are done baking, remove them from the oven and immediately spoon or pipe the lemon meringue evenly on top of each muffin.
Return the muffins to the oven and bake for an additional 5 minutes or until the meringue is lightly golden.
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.