Make lavender syrup first: Combine 1 cup water, honey/agave, and dried lavender in a small saucepan. Bring to a gentle simmer over medium heat, stirring until sweetener dissolves completely. Once simmering, reduce heat to low and continue cooking for 5 minutes. Remove from heat and let steep for an additional 10 minutes.
Strain the lavender syrup through a fine-mesh strainer into a heat-safe container, pressing gently to extract all the flavor. Allow to cool completely (can be refrigerated to speed up the process).
Prepare the blueberry mixture by adding 1½ cups of the blueberries to a cocktail shaker or mason jar. Muddle thoroughly until the berries are completely crushed and juicy. Reserve the remaining berries for garnish.
Add 3/4 cup torn basil leaves to the muddled blueberries and gently press with the muddler to release their oils without completely crushing them. The goal is to extract the aromatic compounds without making the mixture bitter.
Pour in the fresh lemon juice and 2 tablespoons of lemon zest, then add 4-6 tablespoons of the cooled lavender syrup (start with less and adjust to taste).
Add ice to the shaker and seal tightly. Shake vigorously for 30 seconds to combine all flavors and chill the mixture.
Prepare serving glasses by filling them halfway with fresh ice cubes. If desired, rub the rim with a lemon wedge and dip in a mixture of fine sugar and dried lavender for an elevated presentation.
Strain the blueberry mixture evenly into each prepared glass, filling about 1/3 of the way.
Top each glass with sparkling water, pouring slowly down the side of the glass to maintain the beautiful color gradient and preserve carbonation.
Garnish each mocktail with fresh blueberries, a lemon wheel, and a small sprig of fresh basil. For extra visual appeal, add a single lavender sprig or edible flower.