A blue screen is often used for chroma keying in situations where the subject is wearing green, making a green screen impossible to use. It provides a stark contrast to human skin tones.
Beyond video production, a bright blue screen can be used to test the blue sub-pixels on your monitor, ensuring color accuracy and identifying any stuck or dead pixels.
Green is generally better for digital cameras because sensors have more green pixels, resulting in a cleaner key. However, if your subject has green clothing or props, a blue screen is the necessary alternative.
Yes, exposure to blue light suppresses melatonin production, which can make you feel more alert and delay sleep.