I have had a regal blue tang for three years. It is a beautiful fish, however, there are times that it gives me an ulcer!
It is normal (but strange)blue tang behavior to lay flat on the sand or lay down flat next to something like a rock. They do it when they are stressed/scared and always when they sleep.
Mine does change color at night. He becomes pale, and the blue section can look at little white, but not his black section. Yours maybe very stressed.
I do not know how much you know about this fish, but stress is a hugh problem for them. It causes ick, and it does not take much for these fish to get ick. I always call them ick magnets.
Watch closely for white spots and since this is a new fish I hope you are doing something preventative for ick at this time.
Reduce stress. Make sure it have something to hid in or against. Keep other fish from bothering it.
Feed a variety of food and include seaweed select soaked in vitamins like Zoes.
Try not to turn on the lights after they have gone off. I use a dim flashlight to check things out after my light so off.
Good luck, these can be very difficult fish.
Connie
