In a bowl, toss together the blackberries, sugar, and a pinch of salt.
Cover and let sit in a warm place for 2 hours.
Strain the syrup over a bowl, reserving both the liquid and the berries.
Fill a tall glass with ice.
Add the lime juice, ginger beer, and ½ ounce of the fresh blackberry syrup and stir to combine.
Slowly pour in the rum so it floats on top of the ginger beer.
Garnish with a lime round and a small spoonful of the berries.
Serve.