Preheat oven to 375°F. Line a 12 muffin pan with cupcake liners. Set aside.
Separate 36 blueberries and set aside.
Prepare cake mix according to package instructions. Fold in lemon zest. Stir in remaining blueberries. Scoop batter into prepared muffin pan. Bake according to package instructions.
For the frosting
Meanwhile, make the frosting. Place cream cheese and sugar in medium bowl. With an electric mixer, beat until smooth and creamy (or stir vigorously by hand until creamy). Pour into pastry bag with a 1M pastry tip. Pipe a small amount of frosting onto each muffin (or if you don't have pastry bag and tip, just spread onto muffin with butter knife). Add 3 blueberries to each muffin. Garnish with a mint leaf, if desired.