Can a canker sore be black? why?

Possible: If there is trauma to the sore there can be some bleeding into the sore. This will give a dark appearance. Have your dentist take a look as sometimes it might be something else.

Rarely: Canker sores may occasionally turn a bit dark while healing, but not usually black. See your dentist to have it evaluated.

Yes: Cancer sores or aphthous stomatitis is in a lot of cases a recurrent problem, most of people who develop it have genetic predisposition to it. The causes are not well known.

Canker sore: Canker sore are typically rounded white ulceration inside the mouth. They will heal in about 10 days.