Step 1: Place the chilled juice (or water), followed by the frozen mixed berries, and optional sweetener into your Ninja blender jar. Ensure liquid is at the bottom for optimal blending.
Step 2: Secure the lid on your Ninja blender. Start with the 'Pulse' setting for several short bursts to begin breaking down the frozen fruit. This prevents the blades from getting jammed.
Step 3: Once the fruit is partially broken down, switch to a continuous 'Blend' or 'Smoothie' setting. Blend until the mixture is completely smooth and has a thick, icy slushie consistency, scraping down the sides with a spatula if necessary.
Step 4: If the slushie is too thick to blend, add cold water one tablespoon at a time and continue blending until the desired consistency is reached. Avoid adding too much liquid, as this will make it less icy.
Step 5: Taste the slushie and add more sweetener if desired, then blend again quickly to combine.
Step 6: Pour the non-alcoholic berry blast slushie into chilled glasses. Garnish with fresh mint leaves or extra berries, if using, and serve immediately with a spoon or wide straw.