I appreciate the validation, haha. I'm pretty sure we agree on what's supposed to happen (even if the rules wording is a bit iffy) so I'll go ahead and close this.

I could see that, but then it kind of begs the question of why 509.1h has a statement for how a creature becomes "unblocked" that isn't just "if it's not blocked, then it's unblocked":

... An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature...

There doesn't seem to be anywhere else in that section that explains what constitutes an "unblocked" creature.

So, what's the rule that causes it to become unblocked? Is it 509.1h? My impression was that 509.1h only "happens" once during the declare blockers step, but I could be misinterpreting it. I also find it odd that ninjutsu would have its own ruling stating that the creature enters unblocked if that's supposed to be a given.

Or, more specifically, when does Zareth San, the Trickster become unblocked when using his activated ability? It seems like it should work similarly to how ninjutsu does, but there's actually a specific rule (702.49c) that states that creatures entering with ninjutsu are "put onto the battlefield unblocked". It seems obvious that Zareth San should be considered unblocked when entering using his ability since there's no way to assign blockers to him after he enters, but it doesn't actually say that anywhere on the card. Unless I'm misunderstanding, his ability can only be activated after creatures usually become unblocked (509.1h), otherwise you wouldn't have an unblocked creature to swap out in the first place.

Side note: I've checked the Gatherer rulings for Zareth San and Thousand-Faced Shadow (which presents a similar problem), but neither of them seem to have a ruling on this issue. Makes me wonder if there's an oversight somewhere, or maybe I'm misunderstanding rule 509.1h.