Place chicken drumsticks in the slow cooker (remove the skin if you prefer). Season with salt and pepper.
In a separate bowl, whisk together barbecue sauce, orange marmalade, blackberry preserves, red pepper flakes and liquid smoke. Pour sauce over the drumsticks. Cover and cook on low for 4-5 hours, or high for 3-4 hours.
In a small bowl, stir together cornstarch and water until cornstarch is completely dissolved. Skim fat from the juices in the slow cooker and discard, then stir in the cornstarch mixture. Cook until sauce is thickened; about 15 minutes.
(Optional) To caramelize the sauce and get a slight char on the chicken, turn on the broiler in your oven and place chicken, skin side up, on a foil lined baking sheet. Brush with additional sauce and broil for a 5-7, watching closing to avoid burning.
Serve hot, with additional sauce on the side. Garnish with green onions if desired.