Acne can be stubborn sometimes. For those stubborn areas, be really generous with the bp and use the AHA to spot treat as well. You could also try icing the area after washing but before applying any treatments.
Skin darkening in the beginning is almost always due to dry skin. Be sure you are using plenty of moisturizer and add some extra jojoba oil drops at each application. That should help. If it doesn't you may need to reduce the amount of bp you are using in this particular area.