What Blatantly Not Me Nan Means
The call-out, not the question.
The line: "You're blatantly just not me nan" is the wolf's line from the Hoodwinked / Into the Woods scene - the moment the grandmother in the wolf's skin is right there and the wolf, in the same breath, calls it. The line is not "are you who you say you are?" It is "you are not who you say you are, and we are saying it out loud."
The register: it is the "I see right through you" tone. The word blatantly is doing the whole job. It means the disguise was not subtle, was not close, was not almost working. It was a full-face failure and everyone in the room had been watching the seams the whole time.
Where it spread: it became the caption for the moment a person's claim is too thick to hold - the "expert" who gets the basics wrong, the "local" who has never been to the corner of the actual street, the "biggest fan" who cannot name a single song. The line lands because it is not a challenge. It is a read that has already happened.