Among the many “laws” (that is, rules and regulations for right behavior), that God — through Moses — gave the nation of Israel was an unconditional prohibition against drinking blood.
Every Hebrew would have been taught this from early childhood. Breaking this law would have been unthinkable to them, a repulsive act to even consider. Drinking blood was strictly verboten.
Then Jesus came along with his radical teaching that shocked many. He told his followers that they needed to drink his blood. His followers — all Hebrews — were appalled. Viewing his statement as heresy, many turned their backs on him and left.
The idea was so repulsive to them that they were unable to get past the shock of a literal interpretation to consider that it might just have a figurative meaning.
In making this bold statement, Jesus was foreshadowing his sacrificial death. Succinctly, his blood would be spilt as a redeeming, life-restoring sacrifice.
Jesus wasn’t contradicting the laws of Moses. Instead, he voiced his intention to fulfill it.