The pale yellow liquid component of the blood, plasma, is not najis because generally it is not considered as blood which is supposed to be red in colour. Yes, if something comes out of the body which is reddish is colour, it has to be considered as blood and the rulings of the impurity of blood has to be followed.
The sealing of the body with cotton is due to the impure nature of red liquid coming out of the body. However, after Ghusl and Kafan if one sees blood on a part of the Kafan, they have the option of just cutting that piece of Kafan and there is no need of replacing the entire Kafan.
Courtesy: Office of His Eminence Sayyid Sistani