1. Process blueberries in a food processor or blender until smooth, stopping to scrape down sides. Add sugar, lime rind, and lime juice; process until well blended. Pour into an 11- x 7-inch baking dish. Stir in soft drink. Cover and freeze 8 hours. Remove from freezer; let stand 5 minutes.
2. Chop mixture into large chunks, and place in food processor in batches; pulse 5 to 6 times or until mixture is smooth. Serve immediately, or freeze until ready to serve. Garnish, if desired.
To make this a no-added sugar dessert, use 1/2 cup of no-calorie sweetener (such as Splenda) in place of the sugar.
Per 1-cup serving with no-calorie sweetener: Calories 25 (0% from fat); Fat 0.1g (sat 0g, mono 0g, poly 0g); Protein 0.3g; Carb 6.2g; Fiber 1g; Chol 0mg; Iron 0.1mg; Sodium 0mg; Calc 3mg.