Shakespeare's use of blood imagery  is a representation of the overwhelming guilt that Macbeth and Lady Macbeth feel after the murders.  Like the use of weather to enhance the drama of the presence of evil in the early part of the play.  The blood is a constant reminder of the wrongful acts that have been committed. The actions are so vile that they imprint Macbeth and Lady Macbeth with a new identity. They are vicious murders who are now being consumed by their guilt and being dragged into madness.

Macbeth and Lady Macbeth experience total isolation from everything that makes us human.  Their existence is dominated by the forces of darkness they have unleashed. 

Evil, once unleashed tarnishes everything in nature, therefore, a greater act of good is needed to put order back  into the physical world. This act of good comes in the form of Malcolm who claims the throne of Scotland and the death of both Macbeth and Lady Macbeth. Thereby closing the circle of evil they opened with the surrender of their lives and the temporary gain they received by placing their trust in evil forces.  

Once again, Shakespeare challenges us to recognize that if one accepts the invitation of evil, the cost that is paid will be much higher than the reward.

How does Shakespeare use the imagery of blood in Lady Macbeth?

Shakespeare expresses wonderful imagery of blood in Lady Macbeth when she says, "Make thick my blood; stop the access and passage to remorse." During this time, innocent, wholesome people were thought to have thin blood. However, poison is what made a person's blood thick. As a result, Lady Macbeth wanted her blood thick with poison so that she could kill King Duncan without feeling any remorse for committing such an act. Lady Macbeth wanted to trap any feeling of guild and shame among her thick, poisoned blood. She also explains to Macbeth that her hands are the same color as his covered in blood, but she is okay with it. However, she does not want to "wear a heart so white," like him. Meaning, she does not want a bloodless, lifeless heart and be a coward like Macbeth.