Why are my patient's legs turning black?

I'm a opiate addiction counselor and I have a pt with multiple health issues but one of them really scared me his legs are turning black, he is been dx with cellulitis and he has a back problem, couple of herniated discs, what could be the cause for his legs to be like that?

Dear virginia,
It seems rather obvious that he isn't get circulation to his legs...did you not take this in your health classes? Anyway, he must see a doctor ASAP...I hope you've done that before you've asked this question.
Sue...good luck

poor blood circulation