Prepare 4 small (or 2 large) margarita glasses by rubbing lime along the rims and dipping them in a bowl of kosher salt. Fill glasses with plenty of ice.
Using a blender, puree blueberries with about two ounces of lime juice. Strain out the juice through a fine-mesh strainer, and discard the pulp.
Fill a cocktail shaker with ice. Add half of the tequila, orange liqueur, blueberry juice, and remaining lime juice - and shake vigorously.
Pour into two margarita glasses and garnish with lime. Repeat for remaining two glasses, and serve.