TYB
They’re to alert police of someone running a red light. Say, what? When the police come to an intersection, the blue light indicates the cross traffic has a red light.
The police aren’t always able to see cross-traffic signal lights. So, if they observe a car in cross traffic going through an intersection with a blue light lit, the police know that person has run a red light. You could say it is a blue light for a red light.
https://www.motorbiscuit.com/arizona-mississippi-blue-lights-at-intersections-what-for/