Heat the barbeque to medium-high. Season the chicken with salt and pepper. Lightly oil the grill. Grill the chicken 2 to 3 minutes per side, or until the exterior is nicely coloured. Move the chicken to one side of the barbeque. Turn the heat off on that side of the barbecue; leave the other side at medium-high. Cover and cook, turning the thighs from time to time, for about 30 minutes, or until cooked through. While this occurs, place the remaining ingredients, except the fresh or frozen blackberries, in a pot over medium-high. Cook until the jam is melted and well incorporated with the other ingredients. Stir in the blackberries and then reserve on low heat. Once cooked, arrange the chicken on a platter, spoon over the sauce, garnish with rosemary sprigs and serve.