Preheat the Oven & Prep Pan: Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan. For easier removal, you can also line the pan with parchment paper, leaving an overhang on the sides.
Mix Wet Ingredients: In a large mixing bowl, combine the melted butter, packed brown sugar, and granulated sugar. Stir well until combined. Add the egg, vanilla extract, lemon zest, and fresh lemon juice. Mix until smooth and well incorporated.
Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt.
Combine Wet & Dry: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Fold in Blueberries: Gently fold in the fresh blueberries into the batter until they are evenly distributed.
Bake: Pour the batter into the prepared 9x9 inch baking pan and spread it evenly. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean (a few moist crumbs are okay).
Cool & Serve: Allow the blondies to cool completely in the baking pan before slicing them into squares. Dust with powdered sugar, if desired, and enjoy!