Hello and hope you are doing well.
The pleura is the lining of the chest cavity and lungs and where the diaphragm and the chest cavity meet it is called as the ‘costophrenic sulcus’. This space can get obliterated due to fluid or blood. Since the Xray was taken after an injury, it could be due to blood. Since it is mild, it may not cause any problems with breathing.
Hope this helped and do keep us posted.
i am not really sure what you are talking about