There was something on the History Channel about the long strange history of what happened to Lincoln’s body, and I think they mentioned that they embalmed the body to the max when he died because of the planned tour they were going to make with it. Maybe that caused the mumification and darkening of the skin?
Related info ...
The Adventures of Abraham Lincoln’s Corpse
http://io9.com/5898746/the-adventures-of-abraham-lincolns-corpse?tag=abraham-lincoln