Place the frozen raspberries, ice cubes, sugar syrup, and a drop of blue food coloring into the blender.
Blend on high speed until the mixture becomes smooth, icy, and vibrant in color. This may take about 30 seconds to 1 minute, depending on your blender.
Stop the blender and check the consistency—it should be thick and slushy, with no large ice chunks remaining.
Taste the slushie and add more sweetener or coloring if you'd like a more intense flavor or color. Blend again briefly to incorporate.
Pour the vibrant blue raspberry slushie into glasses, making sure to distribute the drink evenly.
Serve immediately with a straw or garnish with fresh raspberries for a fun, colorful presentation.