I thought that maybe when he was a human he dead like that and these are the trace or marks
I think those marks are because he is a god of Dead and let alive someone who should have died
There is a japanese movie, Premonition - in this movie, the main character got ability to see a pieces of newspaper about some sort of accidents that were about to happen, and he could stop them before they took place - each time he did, a small part of his body (starting from his arm) turned dark-blue, almost black. So I'd say that the marks on Caleb's body are caused by the fact that he changed destiny of a human being
ahh yeah those make more sense. i was considering the fact that they only mentioned the marks on ch 4 and not previous chs.
do you think caleb has those marks because he fell in love with a human