Hello,
Since the marks have been present for such a long time,so I don’t think that they are dangerous.They sound like Fordyce spots,marks due to friction rub with clothes or simple hyperpgmentation of the skin.
This darkening occurs when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin.
Treatment involves hydroquinone, azelaic acid, corticosteroids, tretinoin cream and laser treatment.