Bring water to a boil in a kettle and pour over the tea bags in a heatproof pitcher. Steep for 3-5 minutes, then remove the tea bags and stir in sugar until dissolved. Let the sweetened tea cool to room temperature.
Rinse the fresh basil leaves and add them to the cooled tea. Let them steep for about 10 minutes to infuse the herbal aroma.
Remove the basil leaves from the tea, pressing gently to extract extra flavor, and discard the leaves. Pour the infused tea into your blender.
Add the cooled brewed tea, ice cubes, and blend on high until the mixture is smooth and slushy, about 30-60 seconds.
Pour the bright, icy basil tea slushie into glasses. Garnish with a fresh basil leaf if desired, and serve immediately for the best refreshment.
