In a small bowl, whisk the BBQ sauce, grape jelly, garlic, and black pepper.
Place chicken legs in a gallon resealable plastic bag. Pour half of the marinade over chicken legs. Remove air from the bag. Marinate for at least one hour up to overnight.
Store remaining marinade in an airtight container in the fridge.
Preheat the oven to 425 degrees Fahrenheit.
Line a baking sheet with aluminum foil and spray with non-stick spray. Place chicken legs on the prepared baking sheet and baste with reserved BBQ marinade.
Place the chicken legs in the preheated oven and bake for 35-40 minutes or until they reach an internal temperature of 185-190 degrees Fahrenheit.
Remove from the oven and let rest for 5 minutes before serving.