Make the Blueberry Layer:
In a blender, combine frozen blueberries, ¾ cup milk, ¼ cup yogurt, 1 tablespoon sweetener, and a few ice cubes. Blend until smooth and thick. Set aside.
Make the Pineapple Layer:
Rinse the blender and combine frozen pineapple, remaining milk, remaining yogurt, 1 tablespoon sweetener, vanilla extract, and ice. Blend until smooth.
Layer or Swirl:
To create a swirl effect, alternate pouring blueberry and pineapple blends into clear glasses. For a more blended version, you can layer them in the blender and pulse once or twice.
Top and Serve:
Top with whipped cream, fresh fruit, or even a drizzle of pineapple syrup if desired. Serve immediately with a wide straw or spoon.