The black staining effect is not uncommon in wearers of 14K gold jewelery. Pure gold is relatively inert and does not rust. However, 14K gold is mixed with silver, copper, or nickel which oxidize (rust or tarnish). Silver tarnishes black/grey and copper tarnishes green/dark green. Nickel can cause a chemical reaction with make-up causing it to leave dark grey/black mark on the skin. This effect is especially amplified in those allergic to nickel or those living near salt water or in tropical environments which amplifies the oxidation or metal.
