Shakespeare opens Macbeth with a scene rich in imagery that sets the tone for the play. Amid thunder and lightning, the witches are planning to meet on an unnamred battlefield (where they encounter Macbeth.) They sing:
Fair is foul, and foul is fair.
Hover through the fog and filthy air.
This imagery foreshadows the course of the play, in which what it is clear that foulness is at work. Throughout the play, we often see blood, for example, on Macbeth's and his wife's hands after he commits the murders and she the cleanup. This blood comes to symbolize guilt later in the play, when Lady Macbeth, overcome by guilt, is sleepwalking in the famous "out, damn'd spot!" scene. Blood thus symbolizes the actual brutality of Macbeth's actions as well as the guilt that he experiences. Banquo's ghost appears to Macbeth covered in blood, and of course, the dagger that points the way to Duncan's chamber is bloody as well.
Darkness, blackness, and night are often used to symbolize evil, as in the weird sisters' dark gathering, and the act of concealing evil, such as when Macbeth asks for the "stars" to hide their fires so that his "black" desires will not be revealed to the world.