Someone got killed by a bullet in the head. The gunshot entered the right side and came out the left. Five days later, just before getting buried, someone saw the beds of his finger-nails which were totally black!

Was he poisoned as well as shot or is there any other explanation for the black nails?

Thank you very much in advance.

Blood under the skin turns darker and darker with time, and skin and underlying tissue turn brown to black as they necrotise (die), past the moment of death. Was this when he was found, or after autopsy? Blood can easily pool in the fingers depending on how he was lying. Lividity is the pooling of blood in the parts of the body that are against something like the ground or floor.