Hello~ According to a Google search I did, this is what it MIGHT mean: Costophrenic (CP) angle or the costodiaphragmatic recess is one of the review areas in reading a chest radiograph systematically. Normally, the CP angle is acute and sharp. ... Isolated blunting of the CP angle is usually considered to represent pleural disease such as small pleural effusion or mild pleural thickening.
I would,however, ask your GP to discuss the true meaning, as the report said, it could be an artefact, in which case, then it probably isn't too serious. I am not familiar with readings like this, so, I am only repeating what I read, the GP is your best source.
Blunting of the costophrenic angle on a chest radiograph may suggest pleural effusion or thickening. Pleural effusion refers to buildup of fluid in the space between your lungs and chest wall. In your case, this was possibly artifactual so it may not even be real. The rest of your report is normal. Your lungs are clear. Your heart is normal in size.