Combine the rum, Simple Syrup, ginger juice and black currant liqueur in a shaker with ice. Shake until chilled, then strain into high-ball glasses with ice, filling the glasses three-quarters full. Top each glass with some ginger beer and garnish with a lime wedge.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.