To get a half-salted rim, you'll need to start with a tablespoon of salt--but only about 1 teaspoon ends up sticking to the glasses.
Combine 1 1/2 cups water and sugar in a container with a tight-fitting lid; seal. Shake 2 minutes or until sugar dissolves.
Thread 1/2 cup blueberries evenly onto 8 (4-inch) wooden skewers; reserve skewers for garnish.
Muddle remaining 1 1/2 cups blueberries and mint leaves in the bottom of a pitcher. Add sugar mixture, tequila, and lime juice; stir. Moisten half the rim of 8 margarita glasses; dip in salt. Discard remaining salt. Serve margaritas over ice; garnish with a lime wedge and blueberry "stirrer."