Use 16 oz (or bigger) blender.
Make watermelon juice by squeezing/straining 1 cup cubed watermelon through a medium-mesh strainer held over measuring cup. Makes 1/2 cup juice.
Add blueberries, 1 1/2 cups cubed watermelon, watermelon juice to the blender.
Add pinch of sea salt to blender.
Blend until smooth, about 1 minute.