Recently Zombie Research Society member Justin Bates took a look at what might happen to a zombie if its blood stopped flowing and became stagnant after death and reanimation.
Bates found that it’s likely impossible for the undead to present a credible threat to the living if their blood reacts in a similar manner to deceased humans. They would simply not be able to move around well enough to hunt.
“The gravitational pooling of blood in a corpse is called livor mortis. It causes blood to flow towards the part of the body closest to the ground where gravity is most concentrated. As the blood accumulates that area swells and becomes discolored, stretching the flesh to the point of breakage.”
In the case of a zombie that stands up and seeks out prey livor mortis means that all blood inside the body would quickly move to the feet, bursting through the skin and destroying any remaining tissue. The undead menace would be literally walking on bones alone. And those bones, absent of the protective casing of flesh and muscle, would also break apart in short order.
So unless you believe that zombies are short creatures that awkwardly hobble around on leg bone stumps, more prone to falling over than eating brains, it seems clear that something is happening to undead blood to make it behave differently that it normally would in a corpse.