Heat oven to 350°F. Grease and flour a 9-1/2-inch springform pan. Prepare cake mix according to package directions; stir blueberries into batter. Pour batter into prepared pan. Bake 60 to 65 minutes, until a toothpick inserted in center comes out clean. Transfer to wire rack and cool completely.
Cut cake into flower shape with a small knife. Cut a slight hollow about 4 inches wide and 1-1/2 inches deep in center; discard scraps. Place cake on a large serving platter. Slip narrow pieces of waxed paper under edges to keep platter clean.
Tint 3 tablespoons vanilla frosting yellow. Spoon into a small resealable bag and set aside. Using violet icing, tint remaining frosting light violet. Divide into two bowls; cover one with plastic wrap. Use more violet icing from the tube to tint second bowl of frosting a darker shade of violet. Spoon about 1/2 cup into a small resealable bag and set aside.
Use dark-violet frosting from bowl to cover side of cake and two of the six petals. Swirl frosting with the back of a small spoon or a small spatula in an outward motion.
Snip a tiny corner from bag of yellow frosting. Pipe several lines radiating from center of cake. Lightly blend yellow frosting into violet frosting. (Do this while frosting is still wet.) Cover the remaining surface and other side of cake with light- violet frosting.
Snip a tiny corner from bag of dark-violet frosting. Pipe four to five lines of dark-violet frosting onto center light-violet petal. Radiate lines from center of cake; blend into lighter frosting. Outline flower and petals with violet icing from tube fitted with writing tip. Use remaining yellow frosting to add yellow lines as accents. Pile white candies in center of cake. Remove waxed-paper strips.
We recommend using Wilton violet decorating icing.