hmm interesting question really, I m no scientist but look at this way, the earth rotates slowly therefore once it targets with the sun then our planet’s sky becomes blue, and as earth keeps its rotation flow it moves away from the sun therefore the sky darkens no longer blue and as we know it what was a blue sky at daytime becomes black at nighttime. Overall this is an interesting question, and of course this just an point of view and as someone else suggested try google perhaps or maybe your nearby pub. Library.