Pre-heat oven to 325 degrees. Line a rimmed 9"x13" baking sheet with parchment paper, set aside.
In a small saucepan, bring blueberries and lemon juice to a boil over medium-high heat, then lower heat to continue simmering blueberries for 5 more minutes. Set aside in refrigerator.
Mix dry blondie ingredients, then stir in wet ingredients except egg. Once combined, add egg and mix until smooth. Pour into lined baking sheet and spread evenly. Set aside.
In a food processor or NutriBullet, add cottage cheese, lemon extract, egg whites and Stevia. Blend until smooth, then add Greek yogurt, lemon zest and protein powder. Continue to blend until smooth, then tap container against counter top to release air bubbles.
Pour cheesecake mixture over blondie batter and spread evenly. Add blueberry mixture on top of cheesecake by the spoonful, then swirl with a knife.
Bake at 325 degrees for 25 minutes, or until edges are just golden brown and center of cheesecake is mostly set. Allow to cool to room temperature, then refrigerate at least 4 hours before cutting.