Just ruffling through some Darkin lore and it seems like a possible theory. If Shadow Kayn is canonically what would happen if the host was strong-willed enough to vanquish the evil within the weapon, then Poppy's hammer could very well be a cleansed Darkin weapon. Now, granted, we all know that Poppy is the rightful owner of the hammer, it's simply not in the story's best interest otherwise. There also isn't necessarily any gain to her discovering she is the rightful owner. However, Riven and Yasuo have future lore where Riven is confirmed to have killed his master, but it was actually an accident resulting from the blade exploding when shattered. It would be interesting if she learns that the weapon is actually a cleansed Darkin weapon, and she simply has the capability to wield its powers.
Also, Riot, take notes, this could be an opportunity to have a Darkin Poppy skin!