Fill the cocktail shaker halfway with ice cubes and add the tequila, fresh lime juice, simple syrup, and black crème de cassis.
Secure the lid tightly and shake vigorously for about 15 seconds until the mixture is well-chilled and slightly frothy.
Strain the mixture into a rocks glass filled with fresh ice, using a strainer to catch any small pulp or ingredient bits.
Garnish with a lime wedge or a twist of lime peel for a bright finishing touch.
Serve immediately and enjoy the dark, vibrant appearance with the refreshing citrus flavor.