1Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 24 regular-size muffin cups.
2Make and bake cake mix as directed on box for cupcakes, using water, oil and eggs and adding lemon peel. Cool 10 minutes; remove from pans to cooling racks. Cool completely.
3In small microwavable bowl, place 1 cup blackberries. Cover; microwave on High 1 minute to 1 minute 30 seconds, stirring once, until softened. Mash with fork. Place strainer over clean small bowl; line strainer with cheesecloth. Pour berries into strainer; press with back of spoon to remove seeds. Reserve juice; discard seeds and pulp.
4In large bowl, beat butter and powdered sugar with electric mixer on low speed until blended. Gradually add reserved blackberry juice, beating on high speed until frosting is smooth and spreadable. Frost cupcakes. Garnish with additional blackberries and mint.