Are you sure there was no other color added? If so, a black light should help detect it.
In my experience SGC will correctly downgrade a card for such corner staining as displayed on your card, but not to the point of calling it authentic. Granted, my focus is on pre-war, but here is a card from 1916 that shows obvious coloring that could not possibly be intended to deceive:
My example is actually pretty nice compared to about a dozen others from the Green-Joyce set that have sold over the years with much more prominent teal coloring in the corners-- all of which got numeric grades, like this:
