Place the frozen blueberries into your blender. Add the blueberry syrup and cold water or soda to help everything blend smoothly.
Secure the lid and blend on high until the mixture is smooth, thick, and evenly purple. You might hear a slight whirring and see the berries breaking into tiny juicy particles.
Check the consistency—if it’s too thick, add a little more liquid and blend briefly again. It should be icy, yet scoopable with a spoon.
Pour the slushie into glasses, watching the deep purple color swirl as it settles. The texture should be refreshingly cold and slushy, with tiny frozen berry bits.
Serve immediately, garnished with fresh blueberries or a sprig of mint if desired. Enjoy the bright, tart flavor and icy texture right away.
