Add the frozen berries, rum, lime juice, and sweetener (if using) into the blender.
Pour in about 1 cup of ice cubes to help achieve a thick, slushy consistency.
Blend everything on high speed until the mixture is smooth, thick, and slushy, with tiny ice crystals visible.
Check the consistency and add more ice if you prefer it thicker and more frozen. Blend again briefly to incorporate.
Taste the mixture and adjust sweetness or acidity if needed, blending briefly after any additions.
Pour the vibrant berry rum slushie into glasses and serve immediately with a colorful straw or garnish if desired.