The physics of it is that objects can - in a simplistic fashion - absorb and reflect light. The absorbed color is negated. The reflected light is invisibly sent off into space. If, by chance, however, this reflected light is caught by an eye, and subsequently interpreted by a brain, then it becomes color. Like all feathers, blue ones can reflect light in a couple of ways. If the surface is smooth - microscopically smooth - it can allow the pigment inside of the feather to dominate the reflection. This takes place on the molecular level and involves a bit of a prismatic effect.
Prisms are neat. It seems like they can take any light and turn it into a rainbow. But some specifically deformed prisms can retain certain colors and only reflect others. Pretty sneaky, huh? So, even a black-pigmented feather can look blue! Most blue birds have smooth-surfaced feathers, and the absorption and reflection of light takes place inside the feather. The outcome can be quite spectacular. Just picture all the tones of blue on a blue jay. Now, there’s a prismatic work of art - especially given that the bird does not have a single blue feather on its body."
Kinda mind-boggling, isn't it? And what a complex and fascinating world, where blue birds are not truly blue...but we THINK they are. Whoa. That is some weird, wild stuff.
May you all be visited this weekend by the Faux Bluebird of Happiness!