Some people think that black plums cause constipation because they are high in fiber. If you know that the reason for your constipation is fiber, then black plums might make it worse. But if you don’t know what’s causing your constipation, you can try them without worrying about it while looking for the other causes of constipation. For instance, if you have a history of large amounts of caffeine consumption while noticing that your other lifestyle habits are normal (such as regular exercise), then coffee may be the culprit behind your constipation!