I think the Black legion Artefacts are supposed to be treated exactly the same as the
CSM artefacts:
A model can replace one weapon with one of the following.
Only one of each Chaos Artefact may be taken per army.
They just forgot to add bullet point number 3 to the Black Legion, though, and I agree that needs a
FAQ.
3: Does not replact one of the character's weapons.
In
CSM, point 3 is added to the Dimensional Key and the Scroll of Magnus. Items decidedly NOT weapons. I think this can with some certainty be carried over to The Crusible of Lies and the Skull of Ker'ngar, as there is nothing weapon-like about these either.
The Spineshiver Blade, the Hand of Darkness and the Eye of Night are definately weapons, so no question there. These replace a weapon.
Last Memory of Yuranthos is a bit more tricky, as it is not a weapon in itself, but add a Nova power with a weapons profile. My gut feeling is that it is not a weapon.