In a cocktail shaker, gently muddle the blackberries to release their juices.
Add in the tequila, grapefruit juice, lime juice, simple syrup, and some ice cubes. Shake until everything is well combined.
Add the salt to a small plate. Run a lime wedge around the rim of your glass, then press it into the salt to coat. Fill the glass with ice, pour your cocktail in, then top it off with a splash of club soda for that fizzy finish.
Garnish with a grapefruit wedge, lime slice, or extra blackberries.