IMO, your looking at rust. That's not an area that is normalle exposed to a lot of oil but it is an area where water could become trapped and cause some rust. However, just on the off chance it is dried oil residue, soak a cotton cloth with some mineral spirits or WD-40 and see if that will wipe off. If it wipes off and you don't see any pitting or loss of blueing, it's just a stain. If it has rusted, touch it up with some cold blueing and make sure to keep this area waxed or protected with oil.