Add 1 cup blueberries, the honey or simple syrup, and 2 tablespoons water to a small saucepan. Set over medium heat and cook for 4 to 5 minutes, stirring often, until the berries burst and the mixture looks syrupy and glossy.
Remove the pan from the heat. Stir in the lemon zest, then let the blueberry mixture cool for 5 minutes so the flavor settles in.
Place a fine-mesh strainer over a bowl or directly over your pitcher. Press the blueberry mixture through with the back of a spoon to extract as much smooth syrup as possible. Discard the skins if you want a smoother drink.
Pour in the fresh lemon juice and stir well. The mixture should smell bright and citrusy at this point.
Add 1 cup ice to the pitcher, then pour in the cold sparkling water or still water. Stir gently so you keep the bubbles if using sparkling water.
Taste and adjust. Add a little more honey for extra sweetness or a splash more water if you want it lighter.
Fill serving glasses with ice. Pour the drink over the top, then garnish with lemon slices, extra blueberries, and mint leaves if using. Serve immediately for the most refreshing experience.