Measure the crushed ice and place it into your blender.
Mix the blue raspberry syrup with water and stir until fully combined, then pour it over the crushed ice.
In a small bowl, whisk the cornstarch with a little water until smooth and free of lumps.
Bring the cornstarch mixture to a gentle simmer in the microwave or stove until it thickens slightly, about 30 seconds to 1 minute.
Add the thickened cornstarch to the blender and pulse a few times to evenly distribute, giving the slushie a chewy, candy-like consistency.
Blend on high for about 20-30 seconds until the mixture is uniform and starts to look slushy with a slightly thicker, chewy texture.
Pour the slushie into glasses immediately, noting the vibrant blue color and icy, semi-opaque appearance.
Serve immediately with a straw, enjoying the cold, chewy, and icy texture that makes each sip playful and nostalgic.