Preheat oven to 400°F. Purée blackberries in a blender or food processor until smooth. Press through a sieve to remove seeds. Discard solids. Combine 1/4 cup blackberry purée, Dijon mustard, vinegar, 6 cloves garlic, rosemary, thyme, salt and pepper in a medium bowl. Set aside remaining 1/4 cup blackberry purée.

Pierce meat using meat fork every 1 inch. Rub blackberry herb mixture over the lamb, coating the outside completely. Sprinkle remaining 24 peeled halved garlic cloves in a large roasting pan. Place lamb in roasting pan. Let stand 20 minutes.

Roast 30 minutes. Reduce oven temperature to 350°F and continue to roast 1 hour longer until meat thermometer reads 145°F for medium-rare. Remove lamb from pan and let rest for 10 to 15 minutes before carving.

Pour off all but 1 tablespoon drippings from roasting pan. Add 1/2 cup chicken broth to roasting pan and stir with a wooden spoon to loosen and dissolve any browned bits. Pour broth mixture into a blender or food processor and purée until smooth.

Combine garlic broth mixture, remaining chicken broth, remaining blackberry purée and cornstarch in a small saucepan. Bring to a boil over medium heat, stirring constantly; boil 1 minute. Stir in blackberries. Serve over sliced lamb.