The play deals with some controversial topics--murder, the supernatural, the evil side of human nature.  I have always found the animal imagery in the play very intriguing.  Macbeth references animals which typically are not warm, fuzzy, or cuddly...snakes, scorpians, tiger, rhinoceros, bears, etc.  Those who are good in the play reference animals with more positive connotations.  This symbolizes the very nature of humans and of the world...the night Duncan is murdered horses eat each other, owls screech unnaturally, etc. 

The darkness of the play even warrants precautions taken by those who perform it. Most actors are forbidden to say the name of the play inside the theatre for fear of the evil attached to it causing some malfunction in the performance.

In addition, some of the 'brooding' aspect of the blackness comes from one of the themes of the play - human ambition vs. fate and the supernatural.  The play begins with the witches and their predictions - these predictions loom over the rest of the play, being revisited and added to as the witches reappear. The question arises as to whether or not Macbeth has control over the future or are he and his wife a victim of fate or the supernatural.  What would have happened if he had done nothing?  Would the witches predictions still have come true and if so, in what way?  Their sorcery and foretelling set up the tone for the brooding blackness to extend throughout the play.