|MadSci Network: General Biology|
I am very sorry that you had to witness this sight. Without more details about your friend's death it is hard to determine the cause of the top of his head turning black. I am going to assume that the black color that you witnessed was caused either by lividity (the pooling of blood) or putrefaction (rotting). I will list several possible scenarios in hopes of answering your question:
1. If your friend died in the water and his body was not recovered immediately then the typical head down position of a floating corpse could explain his pattern of lividity. Buffeting in the water also commonly produces post-mortem head injuries, which can lead to further bruising of the head which would intensify the discoloration.
2. A fatal car accident involving head trauma can also cause lividity in this area. The trauma, and resulting broken blood vessels, combined with the sudden deceleration during the crash can cause blood to rush to the head and pool there.
3. Your friend could simply have died in a head down position or was moved to a head down position shortly after death.
4. Since you did not see your friend's body until a week after death, it is possible that putrefaction had started to occur, assuming that he had not been embalmed until shortly before or after you saw the body. Putrefaction, or rotting, usually begins in the abdominal cavity, but can occur on any part of the body given the right conditions.
Without more information I can't give you an exact cause for what happened to your friend's body. You can try to find out more about post- mortem lividity, putrefaction, and other things that happen to our bodies after death at the following website:
Try the links in the MadSci Library for more information on General Biology.