The sky isn't blue all the time. Sometimes it's green or red or a number of other colours. And at night, it's black. It's usually blue during the day because water particles reflect the colour blue back into your eyes from sunlight. That's why the oceans are also blue. At night, the sky's black because there is no sunlight. It's as simple as that.
The sky is blue because the sunlight entering the Earth's atmosphere hits the particulate water in the air and refracts, much in the same way a rainbow is created. Due to the frequency of blue light it travels farther and more directly than the other colors of the spectrum. In other words, blue light gets through the atmosphere better than other colors.
sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight...hmmm, well what can you say? heheh!