My Husband's Toes Are Turning Black; What Might Be The

my husband's toes are turning black; what might be the cause of this? He is 62 and has had a knee replacement and a hip replacement. The knee replacement seems to have left him with some nerve damage that is causing the skin on his legs to be red, blotchy and itchy. Now his toes are turning black. I'm not sure they are related but I am concerned.