Add the rinsed blueberries, citrus juice, sugar, and ice cubes into the blender.
Secure the lid and blend on high until the mixture is smooth, thick, and ice is finely crushed; about 30 seconds.
Stop the blender and check the texture; it should be icy and slightly frothy, with a vibrant purple color.
Taste the slushie and add more sugar or citrus if desired, then blend again for a few seconds to incorporate.
Once blended to your preferred consistency, pour the blueberry citrus slushie into glasses.
Serve immediately, garnished with a few fresh blueberries or a citrus slice if desired, and enjoy the vibrant, icy treat.