As you can see the old effect is clearly still existing, as the post-itemazation's spell effect uses a trigger to call for that old effect the dagger had, but as an instant cast effect instead of the 1.5 second on USE effect that it used to have. (And you can see on the old BOED descriptions, that the USE effect was on a 30 second cooldown).
Also note that the old BOED had no item level requirement. Still BoP, but an interesting thing nonetheless. Not sure if that was intended by Blizzard, they later gave it the lvl 49 requirement when they changed the spell effect's behaviour on it.
Blade of Eternal Darkness is using the wrong (post-patch itemazation) stats in-game at the moment.
Old BOED: https://web.archive.org/web/20050215112954/http://wow.allakhazam.com:80/item.html?witem=17780 the spell effect/ID: https://web.archive.org/web/20050302053907/http://wow.allakhazam.com:80/db/spell.html?wspell=21978 in the 1.12.1 database: https://db.vanillagaming.org/?spell=27860
post-itemazation (and in-game version): https://db.vanillagaming.org/?item=17780 the spell effect/ID: https://web.archive.org/web/20060525034852/http://wow.allakhazam.com:80/db/spell.html?wspell=21978 in the 1.12.1 database: https://db.vanillagaming.org/?spell=21978
As you can see the old effect is clearly still existing, as the post-itemazation's spell effect uses a trigger to call for that old effect the dagger had, but as an instant cast effect instead of the 1.5 second on USE effect that it used to have. (And you can see on the old BOED descriptions, that the USE effect was on a 30 second cooldown).
Also note that the old BOED had no item level requirement. Still BoP, but an interesting thing nonetheless. Not sure if that was intended by Blizzard, they later gave it the lvl 49 requirement when they changed the spell effect's behaviour on it.