Blood is thicker than water they say. The purpose for blood in the play is two-fold. It shows the grotesque crime of murder and it illustrates the crime of murder against family members.
Consider the murder of Duncan: "who would have thought the old man to have had so much blood in him" (4.1). Not only is he the step up to the crown for Macbeth, but he is also family. He is the ruler to whom Macbeth pledged his loyalty, but he is also Macbeth's blood relative. Duncan is also a guest in Macbeth's home. All of these bonds were overrun by Macbeth, and so there must be blood to show the full extent of his crime against nature. Both of the Macbeths are covered in the old man's blood, which Lady Macbeth says is cleared with a little water: "A little water clears us of this deed:..." (2.2)
Banquo's ghost "shakes his gory locks" at Macbeth after the new king ordered the murders of Banquo (Macbeth's best friend) and Fleance (Banquo's son). Again, the murdered man shows up with blood on his face, indicating again that Macbeth is guilty of crimes against nature.
Guilt is another reason so much blood is used in the play. Lady Macbeth sleepwalks, still trying to clean herself of the blood of Duncan. She also mentions Banquo and Lady Macduff's murders in her sleepwalking stints, which points to her guilt and to her husband's for committing these unnatural crimes.
Once Macbeth is slain by Macduff, the amount of blood in the play isn't mentioned so much since it is only focused on when we talk of the evil.
Blood has two figurative meanings in Macbeth, both following the central paradox of the play, "Fair is foul, and foul is fair" (I.1). Blood is "fair" when it is linked to the preservation or restoration of life, as in Act I, scene 2, when the news comes to King Duncan (by bloody soldiers) that Scotland has been preserved against revolt, or at the end of the play when Macbeth is finally overthrown. On the other hand, blood is "foul" when it is linked to the growing guilt and madness of both Macbeth and Lady Macbeth, beginning with Duncan's murder and continuing to the play's end.