Did anyone check to see what these coins were stored in?  Perhaps the steel box they were stored in put its stain on the surface.

This sounds like disinfo put out by the American and European central banks to scare people away from gold, or aimed specifically at Russia to hurt its credibility in some way.

Central banks are trying desperately to suppress the price of gold by flooding the market with it, which works temporarily, until the supply dries up and the price rises higher than before.  Bankers do not want common folk empowering themselves with real money, they want people to be at the mercy of the fiat paper money as it becomes more and more worthless.