I'm American, I was born in Arizona, I did my schooling in the US, and I have worked in the US for all of my career. I disagree with your assertion that "thin blue line" necessarily implies support for corruption.
And by the way, so does Wikipedia: https://en.wikipedia.org/wiki/Thin_blue_line doesn't mention this interpretation at all. The closest thing is this sentence, which is really not saying the same thing at all, and at any rate only presenting it as something "critics argue", rather than the settled meaning of the phrase.
> Critics argue that the "thin blue line" represents an "us versus them" mindset that heightens tensions between officers and citizens and negatively influences police-community interactions by setting police apart from society at large.
And yet I've never seen that phrase used other than when cops are defending their colleague who is on video murdering/raping/beating someone innocent, or by those calling for reform who are criticizing the cops covering for each other's crimes.
Even I have seen it used in other senses by Americans, and I've never been to America. AFAICT it has only acquired that sense, at least to the extent it currently has, after #BLM. Might be an age thing, that most of your cultural impressions are of a more recent date than the majority of mine? (And, say, Ted T'so's.)
And it's in a context where some group of people with special power is acting in bad faith to avoid having to follow the rules, and setting up "us vs them" arguments to do so!
And by the way, so does Wikipedia: https://en.wikipedia.org/wiki/Thin_blue_line doesn't mention this interpretation at all. The closest thing is this sentence, which is really not saying the same thing at all, and at any rate only presenting it as something "critics argue", rather than the settled meaning of the phrase.
> Critics argue that the "thin blue line" represents an "us versus them" mindset that heightens tensions between officers and citizens and negatively influences police-community interactions by setting police apart from society at large.