Place the frozen berries into the blender, spreading them evenly around the jar.
Pour the chilled champagne over the berries, allowing them to float and absorb the bubbles.
Secure the lid on the blender and blend on high speed until the mixture becomes smooth, icy, and slightly frothy, about 30–45 seconds.
Check the texture; it should be thick and slushy with small ice crystals. If needed, blend a few seconds longer.
Pour the champagne slushie into chilled glasses, filling them just below the rim.
Garnish with a few fresh berries or a sprig of mint for a fancy touch and serve immediately to enjoy its refreshing sparkle.