It may also be what happens to tarter when it gets far down into the gum. The calculus itself turns black and becomes rock like. This would show through the gums as blackening you describe.
Please go to your dentist first for a through cleaning. It may be all you need. When I had my teeth cleaned my dentist showed me some really black tarter he removed. Once it was gone the darkness in my gumline went away with it.
Best to you,
Gellia
Hi. How long has this problem persisted? When did you first notice it? Call your primary physician, tell them the symptoms and ask what to do. If this is a result of some sort of poisoning, they'll want lab work immediately. If you don't have a primary physician, call your public health dept and ask what you should do. Sometimes, when you have a serious problem that needs immediate attention, you need to remember about Kindergarten's Show and Tell: just show up in the office, show them the problem, and ask what to do. If the Dr.'s office thinks this is a dental problem, then get to a dentist. But, whichever you do, do it immediately. This is not something to put off. Will you please let us know what this is and what was done to correct it" Best regards.