Blood is physical, tangible proof of the evil of one's soul. It symbolizes guilt. Blood is connected to Duncan and the criminals' hands. Macbeth and Lady Macbeth, long after they have washed their hands of the crime, will continue to see blood on their hands and in their hearts.
During the banquet scene, more blood / hand imagery from Macbeth to his Lady: "Scarf up the tender eye of pitiful day; And with thy bloody and invisible hand Cancel and tear to pieces that great bond Which keeps me pale! "
Macbeth says this to Banquo's ghost using blood imagery to describe that which is imaginary (like the dagger), already dead: "Thy bones are marrowless, thy blood is cold;"
And his famous aside, which shows that killing will lead to killing: "It will have blood; they say, blood will have blood: Stones have been known to move and trees to speak; Augurs and understood relations have By magot-pies and choughs and rooks brought forth The secret'st man of blood."