Measure out the crushed ice and place it into the blender.
Pour the blue raspberry syrup over the ice, ensuring all the ice is coated for vibrant color.
Add the fresh lemon juice and a drop of citrus flavoring to the blender to balance the sweetness with a zesty tang.
Secure the lid on the blender and blend on high until the mixture is uniformly icy and slushy, about 30-45 seconds. You should hear the ice crushing and see the mixture turn a vivid blue with a thick, textured consistency.
Stop the blender and check the consistency; if it's too thick, add a splash of cold water or soda and pulse briefly to loosen everything up.
Pour the slushie into glasses, and garnish with a lemon wedge or a colorful straw if desired.