[citruscomputing]

As important context, it gained popularity in response to the Black Lives Matter movement.

[shortstuffsushi]

I think you might be mistaking the "thin blue line" concept with the "blue / all lives matter" in this case, thin blue line is neither new nor newly popular with BLM.

[CRConrad]

Certainly more popular since then; probably swept along by "blue lives matter". Have you seen that black-and-blue version of the American flag, with, what is it, six or seven blue stripes (or lines)? How old is that?