*sword expert here.
As Nobby mentioned, any metal polish like Flitz or Metal Glo should remove the stain if it doesn't come off with normal cleaning and oiling. Polish applied with a rag or steel wool in lengthwise strokes so as not to alter the finish.
I've owned that Dread Pirate Roberts sword, kept it in its scabbard, and never encountered this issue before I sold it--- but it looks like simple oxidation which I've dealt with before. It can happen if any moisture gets trapped in the leather and the sword is stored in such scabbard and not regularly maintained. Did you never oil your sword?
Windlass Steelcrafts (the manufacturer of that blade) uses 1065--1095 carbon steel to make their sword blades, which means they are susceptible to oxidation and rust unlike the stainless steel used in many "wall hanger" replica swords. The good news is that the high carbon steel is a sturdier material for sword length blades and can be easily heat-treated to a tough and resilient spring temper, which is what Windlass does pretty well for its low pricepoint (as compared to other battle-ready swords). The trade-off for having a material that is accurate in performance to real swords (gives a hard edge and a springy spine) is that it rusts more easily than cheaper decorative fare. You simply have to oil the sword to prevent this, giving it a layer of protection from moisture. Windlass sometimes adds a thin clear lacquer on their swords under the pretense of adding a bit of protection from oxidation, but it doesn't do much and it looks tacky.
Anyway, any thin mineral oil will work to prevent rust, like sewing machine oil, gun oil, 3-in-one oil, as long as it is applied to a clean, dry blade using a soft cloth or paper towel so that it forms a very thin layer over the entire steel surface. If the sword is used simply for display, this only needs to be done about once a month if you keep fingerprints and dust away from it. Obviously if the sword is used or handled it should be wiped clean and re-oiled before being put back on display.
A good recommendation for low-maintenance protection is to get some
Break-Free CLP and a soft old t-shirt cut into a rag, saturate part of the rag with the oil, and use that to apply the oil to your blade. Store the t-shirt rag in a plastic bag when not in use and after a while of repeating the process the rag will stay good and oily on its own for quick wipedowns. Break-Free has good long-lasting protection because of the thin teflon bond it creates on the steel surface. I've known people who oiled with Break-Free and then unintentionally left their swords unmaintained for several months that still saw no oxidation when they returned.
It's generally accepted that storing a sword in a leather scabbard is not ideal for the blade (stored in a wood core scabbard is better, stored without a scabbard is best), but it's still unlikely to stain if the sword is cleaned and oiled regularly. The same is true for any knife made from a high carbon steel.
Spending one minute a month maintaining a sword will keep it looking new for years.
Here's a good Care and Maintenence Article for other details.
