1. Preheat oven to 350°F. Line two 15 1/2 x 10 1/2 x 1-inch jelly-roll pans with aluminum foil.
2. Prepare cake following package directions. Divide into pans. Spread evenly. Cut through batter with knife or spatula to remove large air bubbles. Bake at 350°F or 1 minutes or until set. Invert cakes at once onto clean, lint-free dishtowels dusted with confectioners sugar. Remove foil carefully. Roll up each cake with towel jelly-roll fashion, starting at short end. Cool completely.
3. Unroll cakes. Spread about 1 cup blueberry pie filling to within 1 inch of edges on each cake. Reroll and place seam-side down on serving plate. Dust with 1/4 cup confectioners sugar. Garnish with mint leaves, if desired.