you need to fix the normal when you only select the body and overturn it. The reason you cause it just because your head’s normal is different with your body, and if you just select all mesh to overturn the normal you will make the right normal for body turn it back and cause black. So you just need to select body and overturn the normal that will make it right.
for fact i’m not good at Blender but good at Max.
i know how to do with that but i could not go on the Youtube because the wall.
i can fix it if you export your file. that just the easy thing because of the normal.
Max and Maya also can fix it too.
As the same way that Max and Maya do in Blender way.
Blender should be can select the head as element, and just make it reverse can solve the problem. Remember do not reverse whole mesh, just select your head to reverse.
Or you can upload your file, i will try to fix it ASA i see the notification.