I used to be addicted to marijuana, so yes I can say from experience that it does change the color of your lips a darker color. This can be permanent depending on how long you have been smoking but it has to be a long time for it to remain permanent. However long the person smokes is how long their lips will remain dark. If the person quits smoking it can take anywhere from six months to year for the natural color to come back. I know some people who have been smoking for years and who have also quit and their lip color has returned to its natural state. If the people you were speaking of lips were completely black chances are all they do all day long is smoke marijuana.