They’re for cops!

Apparently, the blue light is more visible —especially during daylight hours —than is the regular red light. So a police officer can see them easier, especially at distance or from an acute angle. The blue light is hooked to the same relay on the semaphore as is Red. So if he sees you blowing through that intersection after he spies the blue light switch on, he knows you ran the light.

Good Question: What Are Those Blue Traffic Lights?