Combine blueberries, sugar, and water in a small saucepan and bring to a boil. Reduce to medium-low heat and simmer for 5-7 minutes.
While the blueberries are simmering, use a fork to crush them thoroughly and release all of the juices.
Remove from heat and allow to cool to room temperature.
Pour blueberry syrup through a fine mesh strainer, pressing down with a spoon to press juices through the strainer.
To make your cocktail, begin by combining homemade blueberry syrup, lime juice, gin, and triple sec in a cocktail shaker. Add a handful of ice cubes and shake vigorously for about a minute.
Pour the mixture evenly between two 16-ounce cocktail glasses, straining out the ice.
Add fresh ice to each glass, then fill to the top with seltzer water.
Garnish with fresh blueberries, a sprig of mint, and/or edible flowers.
Notes
*Use fresh blueberries, fresh mint, a wedge of lime, or edible flowers for a garnishIn a pinch, feel free to use hot blueberry syrup, but your drink will become slightly watered down.