Drain the blueberries, reserving 1/4 cup juice.
Combine sugar, cornstarch, and cinnamon in a small saucepan.
Stir in reserved blueberry juice and lemon juice, blend well.
Bring to a boil over medium heat, reduce heat to low, and simmer 5 minutes, stirring occasionally.
Stir in blueberries.
Serve sauce over cupcakes.