Bake shells according to package directions and cool to room temperature. Meanwhile, combine the pomegranate and blueberry juice, 2 tbsp (30 mL) sugar and lime juice in a saucepan over medium heat and bring to a simmer.
Mix the cornstarch with 1 tbsp (15 mL) cold water to dissolve and whisk it into the juice, stirring until it returns to a simmer and thickens. Stir in the blueberries and half the lime zest. Remove from heat and set aside.
In a bowl, mix the ricotta, sour cream, remaining sugar and vanilla until smooth. Divide the ricotta mixture among all tart shells. Top each with the blueberry mixture and garnish with a pinch of the remaining lime zest, if desired. Chill 10 to 15 min. until set. Serve immediately or refrigerate overnight in an airtight container.