Preheat the oven to 350°F. Line a 9x13 glass baking dish with parchment paper and set it aside.
In a stand mixer or with a hand mixer, whisk the eggs on high until very frothy.
Add in 1½ cups granulated sugar and continue whisking at high speed until foamy and thickened.
Add melted butter, lemon juice, lemon zest, vanilla extract, and lemon oil/extract. Beat on medium speed until combined.
In a separate bowl, combine flour and baking powder.
Add the flour mixture to the wet mixture and mix just until combined.
Gently fold in blueberries, just until combined.
Spread the mixture into the prepared baking dish.
Bake blondies for 30 minutes or until a toothpick inserted in the center comes out clean.
To make the glaze, heat ½ cup of blueberries and ½ cup of sugar in a small saucepan over medium heat. You can add a splash of water (1 tablespoon) to help the mixture. Heat, stirring constantly until sugar is dissolved and blueberries have formed a vibrant syrup.
Stir the syrup through a sieve into a bowl with ½ cup of powdered sugar. Whisk until combined, adding a splash of heavy cream or milk to help smooth the glaze into a spreadable consistency.
Spread glaze over warm blondies and allow them to cool the rest of the way.