That's something I think. I'm having fun contemplating magnetism in blades for a couple of reasons:
1. It exists.
2. It exists in folded blades,
3. It exists in tempered blades.
1. is self-evident (and I'm enjoying reading these accounts). As for 2., well, if a sheet of magnetized steel is folded over itself, doesn't it cancel out? What about if it's folded over itself a bunch of times? 3., What happens to magnetization when a blade is differentially heat treated? Since the crystal structure is getting deformed, I'm still puzzling out whether this would affect a magnetic field. Basically, I don't see a straight route from magnetite to a blade without remagnetizing the blade after it's manufactured.
Balanced against all this skepticism are the observations people are reporting here. Perhaps there's a way for a hunk of magnetite to retain its magnetism after it's been folded, tempered, heat treated (and possibly alloyed). Personally, I think the magnetic field came later. As Nechesh points out, it's quite easy to do. As to where those asymmetric fields up and down blades came from, I think that an experiment with some magnets and a non-magnetized sword are in order
Fearn (not Fern, thanks!)