The line, firstly, employs two examples of alliteration. Alliteration refers to the repetition of the same sound or syllable in consecutive words, as in "let light" and "deep desires." In this instance, the technique is used to accentuate the profundity of Macbeth's passion in committing a most heinous act—regicide. Macbeth's "vaulting ambition" is to kill King Duncan and usurp the Scottish throne. In the process, he will override all the rules required for natural ascension.
In addition, the phrase "let light not see" is a metaphor in which light is compared to an ever-present and constant witness that can see all. Furthermore, light is a symbol of goodness and understanding. Macbeth's wish is that light should be blind so that it cannot perceive the evil he is contemplating. In this way, what he plans will remain unknown, and his complicity in the terrible deed he plans to commit will remain undiscovered.
The use of the word "black" is, similarly, a metaphor for the profound evil that has overwhelmed Macbeth. The fact that he calls his desires "black" clearly symbolizes that he means no good. Because these desires are "deep" emphasizes the fact that Macbeth's lust for power has vested itself into his soul and consciousness. He is completely committed to achieving his malicious objective.
The clever juxtaposition of light and dark in the line also exposes the turbulence that Macbeth has been experiencing. The contrast highlights the fact that Macbeth knows that his thoughts are evil and therefore wrong, but he has become so overwhelmed by his ambition that he has allowed malice to overrule reason and virtue.
In the end, Macbeth follows through with his plan and murders his king. He claims the throne and his rule introduces a period of utter depravity in which his paranoia and ruthless thirst for blood becomes the order of the day. Ultimately, it is Macbeth's malice that leads to his self-destruction and almost ruins Scotland.