Why Blue Light? Oceans....
Blue light penetrates water better than other wavelengths.
Isn't that pretty? It's also important. This is most likely the reason why blue light is more important to our circadian rhythms. You can live 20 feet down in the ocean and still see it, whereas most of the red light and other long wavelengths have been filtered out by the time the light reaches you down there.
Why oceans? That's where animals lived when evolution was selecting those who could perform better because they knew the difference between day and night! (Can't handle the evolution idea? Here's my sermon).