Stan
... This late in the war many Federals were also now wearing slouch hats, so presence of one doesnt always imply southern usage as it would tend to have done in 1861 or 1862 era.. western theater being the exception since many western Federals prefered and used the slouch much more than their eastern brethern...

Dead bodies dont always bloat before they change color. Post mortum lividity rate can differ somewhat from person to person. If the "civilian" had been laying face down, then flipped over for the photo... he therefore in the camera viewpoint having a darker appearence than the subject in the foreground would be typical. More times than not dead are commonly seen in photos always laying face up for the camera... He might very well be a person of color, not saying that he isnt. But just stating that there are other issues at hand that could give one the false impression of such. So the other possibilities are feasible... therefore the identity isnt 100 percent conclusive..

Frederick