For I have sworn thee fair, and thought thee bright,Who art as black as hell, as dark as night. (13-14)
Yikes. This is a pretty nasty thing to say to someone—especially when that someone is the object of your sexual desire. When the speaker accuses his mistress of being "as black as hell, as dark as night," he's not just calling her ugly on the outside—he's suggesting that she's morally corrupt on the inside as well. Is he accusing her of being promiscuous? Has she cheated on him? Betrayed him in some other way?