Welcome to the forum. Black-spots and marks take some time to completely disappear. Since you have sinus allergy
get an X-Ray PNS done. Visit an ENT specialist
and get diagnosed and treated. Till then you should try to control your pimples and acne that come on your by controlling your diet. Be very precise as to what you eat. Drink plenty of water, juice and soups. Keep yourself well hydrated.
You should take fiber rich diet, this will help you keep your bowels clean. If you get rid of constipation
your acne problem will slowly stop. Avoid fried, oily food. Take regular bath and exercise till you sweat, it will clean your skin in the most natural way.
You can apply home face packs that suit you. Black marks slowly go away by themselves. However, if you want them clear up faster you can take the help of a Beautician/dermatologist
, there are many therapies and skin lightening creams available that should be taken under expert advice. Take care.