Preheat oven to 350 degrees.
Place tart shells in the cups of a mini muffin pan or on a rimmed baking sheet.
Bake for 4 minutes.
While shells bake, combine granulated sugar, cornstarch, water, and lemon juice in a small saucepan, whisking until smooth.
Stir in the blueberries, and bring to a boil over high heat.
Reduce heat to low, and cook for 2-3 minutes, until the blueberries release some of their juices and the sauce thickens.
Remove the sauce from the heat, and allow to cool to room temperature.
Remove tart shells from oven.
Combine cream cheese, powdered sugar, and 1 tablespoon of milk in a small bowl.
Stir until smooth, adding the additional tablespoon of milk, if needed.
Divide cream cheese mixture evenly among the cups.
Top with a layer of blueberry sauce.
Store tarts in a sealed container for at least 4 hours before serving.
