A big tip-off that you're reading one of Shakespeare's sonnets is the speaker's nasty attitude toward his mistress—it seems like the guy is always criticizing her "dark" features. We know from sonnets like Sonnet 130 that our speaker's mistress has dark hair and a dark complexion. (That's partly why literary critics are always running around calling her the "Dark Lady.") What's weird is that the speaker is always going out of his way to tell her that tons of people think dark complexions are ugly and that light complexions are considered the standard of beauty. (Ever read Sonnet 127?)
That's not all. The speaker of the sonnets also has another obnoxious habit: he tends to associate his mistress' dark skin and features with promiscuity and deception. In the final couplet of Sonnet 147, the speaker tells her that he once convinced himself that she was "fair" and "bright" but, it turns out that she's "black as hell, as dark as night" (13-14).
What the heck does he mean by that? Well, he's talking about her looks and her morals. He once thought she was beautiful and good ("fair" and "bright"). But now, he knows that she's ugly and totally corrupt. That's pretty insulting, don't you think?