The darkening of skin can be multiple causes like tanning,
vitamin B12 deficiency, addisonian pigmentation, acanthosis nigricans, lichen planus pigmentosus, rhiel's melasnosis or several other causes. A proper assessment and examination will help ascertain the proper cause. For loss of glow & dullness - there are antioxidant therapies like glutathione , lasers for rejuvenation, PRP therpy for facial rejuvenation , peels, etc. Kojic cream alone is safe but less effective. You can try using this for a month twice daily on the dark spot. The BEST cream would depend upon the cause. Get an online consult privately with pictures or meet a dermatologist directly for a proper evaluation of the condition. Hope this was helpful.