Place four blackberries and the simple syrup into the bottom of your cocktail shaker.
Gently muddle the berries until they release their juice. You don’t need to pulverize them into blackberry homicide; just break them down enough to get the color and flavor.
Add the dark rum, pomegranate juice, fresh lime juice and Angostura bitters.
Fill the shaker about halfway with ice.
Shake vigorously for approximately 10–15 seconds.
Fill a rocks glass or goblet with fresh ice.
Strain the cocktail into the prepared glass. If you don't want blackberry seeds or pulp in the drink, strain it through a small fine-mesh kitchen strainer as well.
Top with approximately 1½–2 ounces of ginger beer.
Give the drink one gentle stir.
Garnish with two blackberries and, if desired, a lime wheel or strip of orange peel.