From lemons, grate 1 1/2 tsp peel and squeeze 1/4 cup juice.
In medium saucepan, combine 1 1/2 cups blueberries, 1/4 cup sugar and 1 tbsp water. Heat on medium 5 minutes or until blueberries soften and juice thicken, stirring occasionally.
Transfer to medium bowl and stir in 1 1/2 cups blueberries. Set aside.
Prepare lemon syrup: in microwave-safe small bowl, combine 1/4 cup sugar and remaining 3 tbsp water. Cook in microwave on high 1 minute. Stir in 3 tbsp lemon juice and 1 tsp lemon peel.
In medium bowl, stir together nonfat yogurt and remaining 1/4 cup sugar, 1 tbsp lemon juice and 1/2 tsp peel.
In 8 inch by 8 inch ceramic or glass baking dish, arrange half of the ladyfingers. Brush with half of the lemon syrup. Spoon blueberry mixture evenly over ladyfingers. Arrange remaining ladyfingers over blueberries. Brush with remaining lemon syrup. Spoon yogurt mixture on top spreading evenly.
Cover and refrigerate overnight.
To serve, top tiramisu with remaining 3/4 cup blueberries and garnish with mint sprig.