The image of blood is shown through Lady Macbeth's constant action like she is washing her hands. She exclaims that all of the washing in the world cannot get the blood off her hands. This is almost as if she is seeing her guilt on her hands and trying to clear herself of that guilt.
Darkness is shown through the fact that Lady Macbeth seems to be afraid of the darkness. The doctor says that she requires a candle to be by her bedside at all times and refuses to be in the dark.
The idea of sleep, or the loss of sleep is reinforced through this scene as Lady Macbeth is sleepwalking. Right after he commits the murder of Duncan, Macbeth is afraid that he hears whispering in the castle that he has "murdered sleep" and that the house will "sleep no more". The fact that Lady Macbeth is sleepwalking shows that she cannot truly sleep or rest because of the guilt of all the murders.
This scene redeems Lady Macbeth in the eyes of the audience because it shows that she is actually feeling guilt and remorse for the crimes she convinced Macbeth to commit. The fact that she is wrestling so hard with her conscience shows that she understands what she has done, and that type of remorse and guilt can help redeem a character in the eyes of the audience, to see her feel pain for what she's done.
