Hi Rick,
Thanks for the heads up!
If you have any contamination from oils, staining will not occur.
The blade I have pictured, smells kind of like ivory soap. It has been washed and blacked (maybe ink like a magic maker?) but not oiled. Maybe I'll get lucky and not have to work through the oil problem?
Muriatic or Hydrochloric Acid (HCI) could be used to etch the blade, but would do so poorly on its own.
FECL (I assume you mean Ferric Chloride) is also mildly acidic and used widely to etch blades.
I do not think either of these will remove oil. Generally, solvents would be your best approach (you may have to immerse the blade over nite, scrub and repeat until the oil is removed), if you know what solvent would break down or dissolve the oil you are dealing with (you may have more than just natural organic oils, but mineral oil and other synthetic oils, which would likely need a different solvent to efficiently remove it). Unfortunately, you have to know the oil you are removing to know what solvent to use.
Once you are down to bare metal, Potassium Sulfide will blacken the iron. And yes, it creates some H2S (hydrogen sulfide, btw.. it is toxic), the good ole rotten egg smell (it can't be any worse than my wife’s feet, can it?).
That's why I use it outside with plenty of ventilation.
I will show the good, the bad and the ugly (sounds like a movie).