Preheat and prep: Heat your oven to 350°F (175°C). Line a 12-cup muffin tin with liners or lightly grease with nonstick spray.
Mix wet ingredients: In a medium bowl, whisk the Greek yogurt, applesauce, egg whites, vanilla, and lemon juice until smooth.
Combine dry ingredients: In a large bowl, stir together the flour, protein powder, baking powder, baking soda, salt, sweetener, and lemon zest.
Bring the batter together: Add the wet mixture to the dry ingredients. Stir gently with a spatula until just combined. Do not overmix; a few small lumps are fine.
Fold in blueberries: Toss blueberries with a teaspoon of flour if you want to prevent sinking, then fold them into the batter carefully to avoid streaking.
Portion: Divide the batter evenly among the 12 cups.
They will be about two-thirds full.
Bake: Bake for 15–18 minutes, or until the tops are set and a toothpick comes out mostly clean with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack. Cooling helps them set and improves texture.
Enjoy: Serve warm or at room temperature. For a little extra zing, add a light squeeze of lemon over the top just before eating.