Those dark marks from acne are called PIH...Post Inflammatory Hyperpigmentation. They generally fade on their own in 6 to 12 months. The problem is that we often continue to get new acne lesions, which make new PIH marks, so it seems like they never fade. The best treatment is truly to treat the acne so you don't make new marks.
Try a good quality Vitamin C serum. Vit C tends to make skin skin look brighter, and fades PIH a bit, but doesn't bleach skin. Good quality serums are quite expensive ($45-$100/ounce), but you can make it on your own for fairly cheap. There are threads about it on here if you do a search.