That's not a stain, it's patina! It's beautiful!
If you insist you can polish it off, but it'll come back. You could keep it looking bright like a yucky modern stainless blade if you keep polishing it, but it'd save you trouble to buy a yucky modern stainless knife instead.
Another approach is to blue the blade -- see the bluing thread; I just posted there.
-Cougar Allen :{)