When Macbeth hallucinates the dagger before murdering Duncan, he says, "And, on thy blade and dudgeon, gouts of blood, / Which was not so before....

This blood that Macbeth sees on the dagger that he hallucinates, after he draws his own dagger, foreshadows the way in which Duncan's blood -- and the idea of having blood on one's hands -- will become so central to the story.  Macbeth is about to commit an incredibly bloody murder, so bloody that he will be too traumatized to reenter the room to replace the daggers with which he kills Duncan, and his wife will have to do it.  It will be Duncan's blood on his hands that so disturbs him after the murder.  He asks, "Will all great Neptune's ocean wash this blood / Clean from my hand?" (2.2.78-79).  His guilt is so intense that he says all the blood on his hands would turn the green sea red.  On the other hand (pun intended!), Lady Macbeth says, "A little water clears us of this deed" (2.2.86).  For now, she believes that their consciences can be cleansed of guilt as easily as their hands can be washed clean of blood, but she will later have her own hallucination that she cannot wash the blood from her hand, symbolizing how heavily-laden her conscience by her guilt.  The phantom blood on Macbeth's hallucinated dagger initiates the importance of blood as a motif in the play.