What is the term for the example "dark saint" or "humble enemy" that William Shakespeare uses?
In many of Shakespeare's plays he uses figures of speech such as the oxymoron. An oxymoron is a figure of speech is where the write uses to contradictory terms. When examining these pairs, the reader will notice that the two terms are opposites or contrasting terms.
If you look at your two examples, you'll notice that "dark saint" and "humble enemy" follow this reasoning. If someone is a saint, they are usually not considered to be dark just as if you were describing your enemy, you would not normally describe them as being both your enemy and being humble.
One of Romeo's famous lines from Romeo and Julietcontains several of Shakespeare's oxymorons.
Feather of lead, bright smoke, cold fire,
Here Romeo is using contradictory images to show his distress upton learning that his love (Juliet) is also his sworn enemy.