Add tequila, lime juice, and blueberries to the bottom of a shaker. Use a muddler to press the blueberries and release their sweet juices.
Add ice, close the shaker, and shake vigorously for 30 seconds. Add some blueberries to the bottom of your glass and fill the rest with ice.
Serve
Strain the blueberry-enhanced tequila into the glass. Top with sparkling water or Topo Chico.
Notes
Using blueberry syrup will give this cocktail a more concentrated blueberry flavor. You can add ½ ounce of the syrup to the shaker with or without the fresh berries. Totally up to you!