So what do you think this is, if not natural patina? Unnatural patina? Olympic person secretly rubbing black stuff on their medal and posting it on Instagram for views? Do you think the Olympic committee intentionally purchased medals plated in something other than brass that would patina faster? Or do you think it's not brass and some other metal that oxidizes at a faster rate? I think it would take more effort to create a medal that patinas at a faster rate than it would to just create a medal plated in plain old brass or bronze.

Brass left alone in a clean environment will oxidize very slowly, because it's just in contact with oxygen and electrolytes from the air. Metal oxidizes much faster in certain conditions, such as when it is regularly exposed to large amounts of oxygen and electrolytes, such as those found water and acid. Some examples of things that oxidize metals faster include being in regular contact with body oils, salty sweat, or chemicals like sunscreen or body lotion. Do you think any of these substances may have gotten on a medal being worn by an athlete in the summer for a week?

Go get a piece of brass siding from the hardware store. Don't put any sealant on it. Wear it around your neck for a week and see what happens to it.