Hi Zahir, It seems use of garlic over acne led to an irritant reaction and that healed with black marks. Firstly use a anti acne face wash preferably salicylic acid based and a mild water based moisturiser. Consult a dermatologist, so that your skin can be analysed and oral antibiotic or isotretinoin can be started as per the requirement along with topical anti acne creams. This will help not only in reducing acne but to some extent acne induced marks. Salicylic acid or Glycolic acid peels can also be added to reduce acne as well as pigmentation. If the need be Azelaic acid/glycolic acid /kojic acid or triple combination creams can be added to reduce marks. As you can see there are multiple treatment options, however correct treatment can only be suggested once your skin has been analysed. Hope that answers your query. Take care.