Blackberry Vinegar: Soak the blackberries in the vinegar for 3 hours, then pass the mixture through a food mill. Discard the seeds.Chicken: Saute the chicken and then remove it from the pan. Drain off the fat and deglaze the pan with blackberry vinegar. Add the chicken and demi-glace. Bring to a boil. Finish with butter. Remove chicken to serving plates and cover with sauce.