Just some questions to ponder about the Darkin weapons:
Can Darkin weapons be fractured or physically damaged? If they can be, do they have some sort of healing factor that causes them to return to their usual state? If someone were to try to cut one in half and succeed, then proceed to carry one half of it far away, what would happen? Would whichever half happened to contain the "core" regenerate while the other dissolved?
If they can't be damaged in any way like some sort of adamantium or what have you, then that could also make sense. If they were turned into indestructible items that could never release their prisoners, then as a material they would be perfect, but with the drawback being that most mortals would be unable to withstand the force held inside, therefore making them unreliable at best. However, I would tend to think that just because a soul is sealed within that the material itself would not become indestructible. Perhaps the "core," such as the eye on Rhaast's scythe or the glowing red part of Aatrox's sword near the hilt, is indestructible and the weapon follows a pattern similar to what is outlined in the questions asked above.
Of course from what we know as of now we may not have an answer, but I just wanted to know what others' takes on this would be.