Prepare your cocktail glass by rubbing a lime wedge around the rim, then dipping it into the salt or sugar to create a dark, glossy rimming border.
Add ice cubes to your cocktail shaker to chill it thoroughly.
Pour the tequila, black fruit liqueur, freshly squeezed lime juice, and simple syrup into the shaker.
Secure the lid on the shaker and shake vigorously for about 15 seconds, until the mixture is well chilled and begins to froth slightly.
Take a clean strainer and strain the cocktail into your prepared glass over fresh ice, watching the swirling dark color cascade into the glass.
Garnish with a lime wedge on the rim or float a small blackberry for an extra touch of elegance.