What Causes Itching/painful Hives Covering The Body?

I've been suffering with hives covering much of my body for the past 6 days. I've been to urgent care, ER and my dermatologist. Predisone and antihistamines have not cleared it up. Plugs were taken by the dermatologist to try to determine if it is cellular or vascular--no results yet. Some areas are turning black. What comes to your mind? This is very itchy and painful and I'm not able to sleep.