In or At and the nature of relativism
On my way to work every morning I happen to pass a particular billboard expounding the services of a mortgage maid (or whatever the technical term happens to be... loan officer possibly?) On this billboard is a sad attempt at wit wherein the LO has her son standing next to her profile wearing what is presumably his Karate uniform.
Above them both, a caption reads “‘My mom is a black belt at mortgage!’”
My contention, beyond the obvious missing s from mortgage, is that “in” should replace “at”, so that it instead reads, “My mom is a black belt in mortgages!”
I realize if we somehow verbed the word mortgage (and yes, I realize verb itself isn’t a verb), we could use at in a classically technical sense. Consider “I am proficient at mortgaging” as an example. However, the idea of the classification “black belt” makes this null and void as far as I see it. Since we’re speaking of a particular class within an imagined range of expertise at a subject, then “in” becomes the default modifier regardless of a verb or noun ending.
To put it more concisely, since “black belt” is a particular class of status to the relative noun, then there is really no way to use “at” as the correct preposition.
Do I get the black belt IN grammar or am I clearly far too obsessed with this particular imagined injustice to be a well-developed individual.
Thank you in advance,