Blood is known to all of us to represent life, death and often injury. Blood is an essential part of life, and without blood, we could not live. This is known to everyone, and because of this, when Shakespeare uses the symbol of blood to represent treason, murder and death, it is easily understood and fits in perfectly with the ideas we have of blood. Blood is mentioned often in the play and most times in reference to murder or treason. The first sinister reference to blood is in Act 2, Scene 1, when Macbeth sees the dagger floating in...
and both include direct references to blood, again linking blood to treachery and murder.

I think that throughout the play, Shakespeare effectively conveys theme of death, murder and treason through the symbol of blood. Normally, the word blood makes us think about injury and death, being an essential part of life, and the symbol of blood being used in the play is understood by the audience as being essential to life, and in the context it is used, it is a perfect metaphor for death and murder. I think that it is an effective symbol and is used well.