As Lady Macbeth plans to kill King Duncan, she calls upon the spirits of murder to "make thick my blood; / Stop up the access and passage to remorse" (1.5.43-44). Thin blood was considered wholesome, and it was thought that poison made blood thick. Lady Macbeth wants to poison her own soul, so that she can kill without remorse.
Just before he kills King Duncan, Macbeth is staring at the "dagger of the mind," and as he does so, thick drops of blood appear on the blade and hilt. He says to the knife, I see thee still, / And on thy blade and dudgeon gouts of blood, / Which was not so before" (2.1.45-47). However, he's not so far gone that he doesn't know what's happening to him: "There's no such thing: / It is the bloody business which informs / Thus to mine eyes" (2.1.47-49). Of course the "bloody business" is the murder he's about to commit.
"This is a sorry sight" (2.2.18), says Macbeth, looking at his bloody hands moments after he has murdered King Duncan. His wife thinks that's a foolish thing to say, and when she notices that he has brought the bloody daggers from King Duncan's bedchamber, she thinks him even more foolish. She tells him that he must take the daggers back, place them with the King's sleeping grooms, and smear the grooms with blood. Macbeth, however, is so shaken that all he can do is stand and stare at his bloody hands, so Lady Macbeth takes the daggers from him. When she goes to do the job she thinks he should do, Macbeth still stands and stares. He asks himself if all the water in the world can wash away the blood: "Will all great Neptune's ocean wash this blood / Clean from my hand?" And he answers his own question: "No, this my hand will rather / The multitudinous seas incarnadine, / Making the green one red" (2.2.57-60).
In contrast, his wife thinks his obsession with blood shows that he's a coward. She dips her hands in the dead King's blood, and smears the grooms with that blood, then tells Macbeth that "My hands are of your colour; but I shame / To wear a heart so white" (2.2.61-62). She means that now her hands are bloody, like his, but she would be ashamed to have a "white" -- bloodless and cowardly -- heart like his. She leads him away to wash his hands, and she seems quite sure that "A little water clears us of this deed" (2.2.64). Ironically, when she later goes mad, she sees blood on her hands that she cannot wash away, no matter how much water she uses.
Telling Malcolm and Donalbain of their father's murder, Macbeth says, "The spring, the head, the fountain of your blood / Is stopp'd; the very source of it is stopp'd" (2.3.98-99). Here, the primary meaning of "your blood" is "your family," but Macbeth's metaphors also picture blood as a life-giving essence.
In this scene, the last mention of blood comes from Donalbain, who says to his brother, "the near in blood, / The nearer bloody" (2.3.140-141), meaning that as the murdered King's sons, they are likely to be murdered themselves.
Moments later, Macduff enters and Ross asks him, "Is't known who did this more than bloody deed?" (2.4.22). The deed is "more than bloody" because it is unnatural. King Duncan was a good and kind man whose life naturally should have been cherished by everyone