It's not "bad" for you per se..

Blue light, or light frequencies that is on the higher end of the light spectrum suppresses our body's ability to produce Melantonin, the hormone responsible for sleep cycle..

I mean it only makes sense that our evolutionary bodies uses light as a function to determine sleep cycle since for millions of years, our hunter-gatherer ancestors rely on sunlight cycle to survive, day=awake and night=sleep. Since blue light only exist naturally from the sun (there were no LED in the days of our cave dwelling nomadic forefathers),

Since blue light makes one harder to fall asleep, the lack of sleep is bad for you. Blue light itself is not that bad, it's allowing it to fuck up our sleep schedule (i can say that here, blockchain rocks!) that's bad for you.