Yes around a week before a shed they go a dull dusky shade, depending on their original colour it can indeed have a very pale blue caste, the eyes go cloudy and they generally look as miserable as a snake could get. They can get a bit snappy at this stage so I would avoid handling them. All instincts tell you that your snake is dying (I bet many a snake vet has made a fortune seeing snakes in a blue phase!) they then slowly return to normal colour (the skin loks no different than before the blue phase!) then, about 3-4 days later hey presto they crawl out of the old skin. Just before they go into a blue phase you may notice they crawl into their water dish, I personally give the snake and viv light misting with water every few hours to keep the humidity nice and high. A good shed come of in almost entirely one piece, check the eyes scales have come off and the the tail tip, it is a amazing site if you are lucky enough to witness it.