Shakespeare's Dark Lady is a black haired and raven-eyed 'mistress' and the subject of Sonnets 127-154, often known the 'Dark Lady' or 'Dark Mistress' sequence. Her sudden and comparatively brief appearance (the 'Fair Youth' sequence before it is four-times its length) as subject marks a dramatic change in tone and content.
Sonnets 1-126 are addressed with high-minded, but strictly platonic passion to a young unnamed aristocratic man, extolling his noble character, his classic 'fair' (light-skinned, as was very much in fashion at the time) beauty and unimpeachable virtues. Over the course of these 100+ sonnets, 'the poet' (Shakespeare's narrative proxy) describes in appropriately florid detail many of his subject's finest and noblest qualities. Probably the most famous example is Sonnet 18,
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate;
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date;
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd;
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
Here the poet suggests that he could compare the young man to warm and agreeable summer day, but that he will not because it would do him a disservice. Such a description would not do justice to his many and impressively various virtues and beauty: such a day would be too hot, too bright, too short and too harsh. A better comparison, the poet says, would be to commemorate him in his poetry, as he will live on unchanging and immortally in the minds of anyone who reads his sonnets. This is high praise, indeed.
Compare such ephemeral and noble sentiments with Sonnet 130, address not to the Fair Youth, but the Dark Lady:
My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red;
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head;
I have seen roses damasked, red and white,
But no such roses see I in her cheeks;
And in some pérfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound.
I grant I never saw a goddess go;
My mistress, when she walks, treads on the ground.
And yet, by heaven, I think my love as rare
As any she belied with false compare
Her eyes do not blaze like the sun, her complexion is lacks luster and her voice is pleasing, but hardly beauteous. The poet clearly appreciates her, but she is no goddess. That she walks upon the ground is symbolic of the very physical and base human nature that the poet appreciates. Where Shakespeare's poet loves the young man for his unearthly virtue and transcendent qualities, he loves "My mistress" because she appeals to his more earthly desires and base wants. The Fair Youth is everything the poet appreciates and thinks should be commemorated for the ages concerning the very noblest of human ideals, but Shakespeare's Dark Lady is symbolic of everyday desire and earthly lust.