Depends on whether they want to be spotted. Sometimes the point is to be obviously intimidating and try to divide the protesters over throwing someone out. Other times it's to be an obvious plant so that after getting rid of them the protesters think they're no longer infiltrated and overlook the less obvious one. And sometimes it really is just incompetence.
Another thing to look for is a cell phone standing up in a shirt pocket with the camera facing forward.
Could also be used as an identifier for other officers in case they get into an altercation and need to signal “hey I’m on your side”.
I am not sure if they were being coy, but I saw another user say “color of the day” meaning that it might not just be blue, and that they could be cycling colors to obfuscate detection from protest groups on multi day protests.
Thanks did not expect it to be such a loaded thing:
"The "thin blue line" symbol has been used by the "Blue Lives Matter" movement, which emerged in 2014 as a rebuttal to the Black Lives Matter movement, and gained traction following the high-profile homicides of NYPD officers Rafael Ramos and Wenjian Liu in Brooklyn, New York.
The "thin blue line" has also been associated with white nationalists in the United States, particularly after the Unite the Right rally in 2017, who fly Thin Blue Line flags at their rallies."