Simmer blueberries, sugar, lemon zest, and 1 1/2 cups water in a saucepan over medium heat.
Stir until the sugar dissolves. Cook 7 to 8 minutes, gently mashing berries as they burst.
Strain through a fine-mesh sieve into a pitcher, pressing to extract juices. Discard solids and cool 5 minutes.
Stir in lemon juice, 3 cups cold water, and a small pinch of salt. Taste and adjust tartness with more water or lemon.
Fill glasses with ice. Pour in blueberry lemonade and, if using, top with a splash of sparkling water.
Give a quick stir and serve immediately, chilled and fizzy.