1. In a medium saucepan, simmer the first 6 ingredients (through crushed red pepper) until reduced by half and just syrupy (15-20 minutes). Reserve 2 tablespoons of sauce for brushing on chicken; cool the remaining sauce slightly, then gently toss with blackberries.
2. Preheat grill to medium-high. Place chicken on grill, and cook 5 minutes per side. Brush chicken with the reserved sauce; grill just until chicken is glazed. Serve with blackberries.