What is good to get rid of dark marks leave by scars?

Not much. These are best prevented and hard to get rid of. Some creams, hydroquinone, may help. Dermabrasion can help but is tricky. Lasers may help. Avoid sun to area.
Chemical Peel. All of the mentioned methods (hydroquinone, ipl, sunblock, etc) are important. One other treatment is chemical peeling. Peels such as cosmelan and vi peel are quite good at reducing pigmentation and are less risky than ipl on darker skin.
Combination. The ideal is to avoid with the use of sunblock. Once it has occurred, it may get better with time. A trial with hydroquinone, retinoids, and/or ipl should improve the appearance.
Fraxel laser. Fraxel laser with bleaching cream can help. If the scar is raised, active fx can be sued for evaporate extra tissue.