Blueberry Buttermilk Sorbet

Mmmmmm... the perfect ending to a tempo run:

Ingredients:
1 package (12oz) frozen blueberries
1/2 cup frozen white grape juice concentrate
1/3 cup sugar
1 tsp lemon rind
1 cup buttermilk

Directions: 

In a small saucepan, combine the blueberries, frozen juice concentrate, sugar and lemon rind. Bring to a boil over high heat. Reduce the heat to medium-low and simmer for 5 minutes, or until the blueberries are thawed and begin to pop. Remove the pan from the heat; set aside to cool. In a food processor or blender, combine the buttermilk and the blueberry mixture. Puree until smooth. Transfer the mixture to a metal or plastic container. Cover and freeze for 4 hours or overnight. Remove from the freezer and break up the mixture with a knife. Process briefly in a food processor or blender. Return to the container, cover and freeze for at least 30 minutes before serving.