The allusion to Macbeth refers to obsession with blood and guilt.
An allusion is a reference to history or another literary work. Allusions to Shakespeare are common, as
Shakespeare had a great influence on future writers.
The speech being referred to is the famous sleepwalking soliloquy scene from Act 5, where Lady Macbeth succumbs to guilt over causing the king’s death and creating the monster her husband has come. Lady Macbeth sees the blood on her hands, metaphorically, and she cannot clean it off.
Out, damned spot! Out, I say! One–two—
why then ’tis time to do't. Hell is murky. (Macbeth, Act 5, Scene 1)
Despite the fact that this allusion is most easily recognizable as traced to Lady Macbeth, the soliloquy where Macbeth echoes her words also could be the reference in the poem.
Out, out, brief candle!(25)
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.(30) (Macbeth, Act 5, Scene 5)
There is less of guilt than recognition in the poem, when blood is described. It is an acknowledgement that life is pointless, and there is nothing that can be done.
No one believed. They listened to his heart.
Little—less—nothing!—and that ended it.
No more to build on there. And they, since they
Were not the one dead, turned to their affairs. ("Out, Out-")
A boy is dead, and everyone goes about their business? It seems like Macbeth was right. There is nothing to life but a “walking shadow” that means nothing.