It seems odd to consider that Thor undergoing a dark upgrade would mean trouble for Loki as far too often the opposite is true, though that is exactly the scenario the Avengers found themselves in during one particular storyline–one that introduces a version of Mjolnir that holds a specific power the original does not.
Mjolnir is Thor’s hammer which was given to him by his father, Odin, after being forged from raw Uru metal and enchanted by Odin himself. In essence, Mjolnir is one of the most powerful weapons in the universe, and Thor has wielded her for more than a thousand years. However, even with all of Mjolnir’s strength, Loki has time and again found ways to undermine the raw power the hammer holds through his powerful, dark magic. Loki can create illusions, conjure demons, and cast spells that Thor cannot overcome with Mjolnir alone. A perfect example of this is the story of the Avengers’ formation in their first comic book issue where Loki attacks the Earth and proves to be too much for Thor to handle alone, so all of Earth’s Mightiest Heroes answered the call, and together they fended back the magical fiend. In doing so, Thor and the Avengers made up for Mjolnir’s shortcoming against magic as there is no known way to use the hammer to break certain spells and curses–a shortfall not shared by Mjolnir’s dark counterpart.
In Secret Avengers #34 by Rick Remender and Matteo Scalera, the Avengers of Earth-616 travel to Earth-666 to steal an artifact known as the Orb of Necromancy as it held the power to bring people back from the dead. When they got to this universe, the Avengers found that they had dark counterparts that were something of ‘Halloween Special’ versions of themselves. There was a werewolf Captain America, a vampire Wolverine, a Frankenstein’s monster Punisher (which unfortunately also existed in 616), and a mummy Thor. While Earth-666 Avengers were pretty much played up as jokes, ‘Mummy Thor’ proved to actually be a serious threat, and it was all thanks to his hammer: Rinlojm.
Mummy Thor’s Rinlojm is More Powerful than Thor’s Mjolnir
As shown in this issue, Rinlojm holds a power known as the Black Curse which, as described by the magically-inclined Captain Britain, is essentially anti-magic. What this means is that Rinlojm can render practically any magic user totally powerless–and a number of Thor’s villains happen to be magic users–including Loki. If Thor had the power to wipe away Loki’s shield of magical illusion with the point of his hammer, followed swiftly with a crushing blow by the very same weapon (as Rinlojm is presumably just as devastating as Mjolnir in that regard), then it is safe to say that Loki would be face-to-face with his worst nightmare every time he went up against Thor.
If Earth-616 Thor had Rinlojm, he’d not only never have to worry about Loki again, but also some of Thor's other biggest threats like Malekith or Enchantress as Rinlojm would totally negate their magic and leave them essentially powerless. However, even if Rinlojm would be useful against more than just Loki, it is Loki who has proven himself to be the biggest thorn in Thor’s side, and so it would be Loki who’d suffer the most from this dark, multiversal upgrade of Thor’s Mjolnir.