Back when I was into Japanese swords there were stories about blood thirsty blades, particularly blades that had already seen combat or were tested. By tested I mean that some were inscribed on the tang "this blade cut through three men with one stroke" kinda tested.
Apparently to test a blade they would stack up prisoners and a swordsman would give it a go.
I took all my blades to the local temple so a priest could 'bless' it.
I remember a story about a sword that injured it's owner so much that the person eventually sold it.
The mysticism behind Japanese swords(fireballs shooting out of blades and stuff like that), particularly koto blades, is strong; I don't actually believe it, but I wouldn't go so far as to doubt it.
BTW, I just wanted to add: if you've ever seen a high quality older Japanese blade, I swear those things have an eerie glow. The polish and the metal...in Hawaii we call it chicken skin, something about it, je ne sais quoi.