Get out the hydrogen peroxide, because this play needs it: there's blood all over. From the bleeding Captain in the beginning to Macbeth's bleeding head at the end, literal blood drips from every page. But in our view, it's the imagined blood that really counts.
When Macbeth considers murdering Duncan, he sees a floating "dagger of the mind" that points him in the direction of the sleeping king's room (2.1.50). As Macbeth wonders if his mind is playing tricks on him, the dagger becomes covered in imaginary blood, which anticipates the way that very real daggers will be soiled when Macbeth murders King Duncan.
But where does this dagger come from? Did the witches conjure it up? Is it a product of Macbeth's imagination? Is Macbeth being tempted to follow or warned not to pursue the hallucination? Given what happens later, we're tempted to say that it's Macbeth's own vision, an externalization of his guilt.