The lights, themselves, aren't always illegal. Some of them are. The ones that are obviously emitting blue light are definitely illegal, but the ones that are just so bright that they "feel" blue are typically completely legal.
The problem with those lights is that they're often not installed properly. A lot of inexperienced car owners will install HID lights without the proper housing device, causing the lights to point at an incorrect angle (causing the blindness when one of them tailgates you) and making it so that they don't disperse the actual spectrum of light properly (causing the blueness).
It's often further exacerbated by the fact that the people who do these shoddy installations are the same people who drive pointlessly lifted vehicles and like to really ride your ass at night, blasting that laser beam of blue death straight off your rear-view and into your retinas.
Unfortunately, enforcement on these things is basically nonexistent.