I think that one heavily implied option is that, like Stormbringer, Frostmourne, Armageddon's Blade or (for a non-evil example) at least some portrayals of Excalibur, the "pact" is with the blade itself; while the circumstances of the blade's creation may influence the rules and intents heavily, the core relationship is between the blade (which lends the user not just its steel but its power) and the user (who lets the blade claim himself, and change himself).

That said, it's not that well-supported by the mechanics. And apparently someone in WoTC realised that, too; hence the incredibly vague write-up that simultaneously tries to play up the "blade itself is a patron" angle, the "blade is a (meta)physical representation of the pact, which is made with any non-descript entity" angle and the "shadowfell edgy something we just want to reference Blackrazor even though actual Blackrazor doesn't work like that" angle.

Just pick something that makes sense.