@protoaddct I would consider this more reactive, but regardless, this effect has been a white effect with Containment Priest and Hallowed Moonlight type cards (or artifacts). I have never seen blue stop creatures from entering the battlefield if they are non tokens if they weren't on the stack. Please, correct me if I'm wrong there if there are similar type of effects in blue, I just don't recall.
I'm not mad that this is in blue, I just wouldn't have expected it. This is, though, giving blue more tools than I would have expected.
Blue already has a litany of tools they should not have, and for the most part does everything every other color does in some shape and form, unlike say black which still really cannot directly deal with enchantments.
Even if the exact wording is different, exiling things has never been directly related to a single color. White had swords to plowshares, red had disintegrate, blue had counters that exiled, black had oubliette. All this is in effect is blue countering a specific effect in the most elegant way without the card having a ton of lines of text on it. I find it to be very much within their wheelhouse because it prevents/preempts the creature entering the battlefield, which also preempts ETB effects from triggering, akin to countering.
It probably shouldn't be blue for power reasons, let alone a merfolk wizard. Kinda reminds me of the argument that snapcaster should have been red (it should have.) Would much rather have seem them do a goblin or a vampire or something, but it fits thematically with the color pie since blues color pie has long been "we are the best color in magic that interacts favorably with the other best color in magic, artifact"