Mix together cornstarch and water in a small bowl. Add 6 ounces of blueberries, sugar, and cornstarch mixture to a small saucepan. Place over medium heat. Cook for 2-3 minutes or until sugar dissolves and blueberries soften. Smash blueberries with a potato masher to create a sauce. (Leave some big pieces.) Set aside to cool to room temperature.
In a large mixing bowl, stir together Cool Whip. marshmallows, almonds, remaining 6 ounces blueberries, and blueberry sauce until combined.
Portion fluff into dessert cups or bowls. Top with extra blueberries and almonds if desired.
You can make this recipe up to 6 hours ahead of time and store it in an airtight container in the fridge.