To prepare panna cotta, coat 8 (6-ounce) custard cups with cooking spray. Sprinkle gelatin over whole milk in a small saucepan; let stand 10 minutes. Cook milk mixture over medium-low heat 10 minutes or until gelatin dissolves, stirring constantly with a whisk. Increase heat to medium; add 1/2 cup plus 2 tablespoons sugar, stirring with a whisk until sugar dissolves. Remove from heat. Add buttermilk and rind, stirring well. Divide mixture evenly among prepared custard cups. Cover and chill at least 5 hours or up to overnight.
To prepare sauce, combine the apple juice, 1/4 cup sugar, and lemon juice in a small saucepan. Bring to a boil over medium-high heat; stir until sugar dissolves. Reduce heat to medium; stir in blueberries. Cook 8 minutes or until blueberries are warm and begin to pop. Cool sauce to room temperature.
Place a dessert plate, upside down, on top of each custard cup; invert panna cotta onto plates. Serve with sauce. Garnish with mint sprigs, if desired.