Black smoke rings can have a variety of causes. To name a few, they can be caused by the explosion of an electrical transformer, a home-made trash can smoke ring launcher, fireworks tests, or other pyrotechnics. They generally do not occur naturally and are nearly always the result of something man-made.
There are many potential explanations for sightings. We recommend eliminating the most common and mundane before jumping to less probable conclusions or you submit a report.