Green blood
 

Hi,

In La increíble y triste historia de la cándida Eréndira y su abuela desalmada the grandma in question turned out to have green blood:

Ulises asestó un tercer golpe, sin piedad, y un chorro de sangre expulsada a alta presión le salpicó la cara: era una sangre oleosa, brillante y verde, igual que la miel de menta.

Is it merely el realismo mágico, or a hint on some tradition?

It's realismo mágico.
Some kind of symbolism is definitely present though. Color green is often related to evil things like envy or the eyes of the devil, for example. The fact that her blood is not red, like all human blood, shows probably why she was so evil.