I apologize for the generally snarky responses, and the name thing. Usually quick searches turn up the NYPD Commissioner and LA Chief Parker. Parker, you may want to understand, sold "policing on the cheap" to Los Angeles. They were proud to have a third of the per capita staffing of New York and Chicago (they are still way below), replacing quantity with racist brutality and military tactics (SWAT, helicopters, etc.).

But "the thin blue line" seems to have come from the US Army's translation of the British "thin red line" (their colonial troops in the 1850s . It shows up randomly in mentions of the "Indian Wars," and then in US Counter-insurgency efforts in 1900 Philippines. US troops wearing blue instead of the British red.

It was Philippine-American War commanders who "professionalized" US police after that war, adopting a militarized concept that any groups who were "different" were an insurgent enemy. Even those selling the flag (and phrase) date it to 1900 https://www.flagsofvalor.com/blogs/news/the-thin-blue-line