In a medium saucepot, place the ingredients for the blueberry syrup. Bring to a gentle boil over medium heat. Cover, reduce heat and simmer for 15 minutes or until berries burst.
Using a mesh strainer, strain syrup into a measuring cup. Place in the refrigerator until cool. Makes about 1 cup of syrup.
Make the cocktail:
In a cocktail shaker, muddle the mint leaves. Add the vodka, blueberry syrup, lemon juice and a handful of ice. Shake for several seconds. Strain into a glass filled with ice, then top with seltzer water. Enjoy!