Combine berries, vinegar, whiskey, mustard, lemon juice, honey, brown sugar, ginger, shallot, and cayenne pepper in a blender; blend until smooth.

Place chicken in a resealable plastic bag. Add 1/2 of the berry marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator, 4 hours to overnight. Reserve the remaining berry sauce.

Remove chicken from the refrigerator. Heat oil in a large skillet over medium-high heat. Place chicken in the hot skillet and brown on both sides, basting occasionally with the extra marinade. Cook until chicken is no longer pink and juices run clear, about 15 minutes; discard any remaining marinade. Remove from the heat, and let rest for 3 minutes.

Meanwhile, pour the reserved berry sauce in a small saucepan and bring to a simmer over medium-low heat.

Slice chicken into 1/2-inch thick slices and spoon warmed berry sauce over top.