Sprinkle baby back ribs with pepper and place in a slow cooker. Pour 1/3 cup teriyaki sauce on top. Cover and cook until tender, 4 hours on High or 7 hours on Low.
Cut ribs apart. Combine remaining 1/3 teriyaki sauce, balsamic vinegar, and garlic in a saucepan over medium-high heat. Simmer uncovered for 5 minutes. Brush sauce over ribs.
