Electrum only really tarnishes in the presence of sulfur. At which point the silver parts blacken.
Or dark sorcery, which is more likely in the current case.
As ancient Acheronian coins, they have value only to specific people.
The treasure hoards we are making are to attract ~spoiler redacted ~
And it seems they only care about Gold, Silver, (common currency) and some objects of art.
As copper is frequently noted as common currency in the Hyborian Age, it would be a good addition.
The Obols are more of a collectors item, good on the antique roadshow, less so for mega Mart.