What can cause a black spot on a chest xray?

Bulla: A typical american display of chest x-rays is white bone and dark lungs (british are opposite). Black spots would typically be air cysts or bulla. These can be manifestations of prior infections or emphysema.

Air: Most abnormal spots seen in lungs are white. When air gets into soft tissues such as mediastinum, so called pneumomediastinum, or dissects into neck , subcutaneous emphysema, in pleural space, or in pericardial space can be noted as black in form of air collections.