Rather than the Masamune drinks the blood of the unclean sword, I think it absorbs negative energy(like the aura of a person) than the "sword drinks blood theory." Since the Masamune is suseptible to absorb energy, it's more likely that it absorbs negative energy due to the fact that it is made of dreamstone. It's highly probable that whoever stole the Masamune must be pure evil.
Another thing, since that the one who stole the Masamune wields a scythe, I think that Lynx is the likely suspect.
Actually, that is not the case at all. Masa and Mune clearly say that it is not who wields the sword, but how it is used. Someone that is evil but uses it for good purpose would not taint the sword, whilst a Crono who butchers people, even with good intentions, would serve to corrupt it. As such, it cannot be the energy of the wielder it absorbs, but rather of the events that it partakes in. Rather like a human being, in fact. Somone the is involved in a bloody war, no matter how just people might claim the cause to be, can be driven mad and violent by witnessing the actions of war.
Also, I must agree with all others that is is merely the blood, and not a scythe blade, that is shown in shadowy aspect in the cutscene. Heck, listen to the fighting sound - those sould like swords clashing. I don't even know if a sword and a scythe could be used against each other and, if they are, likely as not the scythe wielder would take the blows of the sword on the shaft, and and sword wielder woudl deflect the scythe by striking the shaft (likely attemping to break it there, too, as that would be its weakest part.) As such, it seems to be more of a sword battle.
One thing, though... who is to say that Janus would even use a scythe later? Don't you think that Janus the high prince of Zeal, chief Sorcerer of the Mystic tribes, could be best with a sword in near encounters, surpassing with a bow, perfect in the balancing of a thrown spear? He has that dread weapon as more of a symbol of terror, less a useful weapon - if anything, its best aspect would be anti-cavalry, as Clopin does in the Hunchback. But I think it strange that strong soldiers couldn't be versed in whatever they put their hands to.
