In the old age black was not counted fair,
Or if it were, it bore not beauty’s name;
But now is black beauty’s successive heir,
And beauty slander’d with a bastard shame:
For since each hand hath put on Nature’s power,
Fairing the foul with Art’s false borrowed face,
Sweet beauty hath no name, no holy bower,
But is profan’d, if not lives in disgrace.
Therefore my mistress’ eyes are raven black,
Her eyes so suited, and they mourners seem
At such who, not born fair, no beauty lack,
Sland’ring creation with a false esteem:
Yet so they mourn becoming of their woe,
That every tongue says beauty should look so.
In plain English
In earlier times, dark skin was never considered beautiful, or if someone called it beautiful, they didn't really mean it. Now black has become beauty's rightful successor, while genuine beauty gets smeared as false and shameful. People have taken nature into their own hands with makeup and artifice, painting over what's naturally ugly with a fake borrowed face. Real beauty has lost its name and sacred meaning — it's profaned, disgraced, barely existing anymore.
So my mistress's eyes are black as ravens, and they look perfect for the job — they mourn at all those women born plain who've learned to fake beauty anyway, slandering creation itself by claiming they're naturally beautiful when they're not. Yet her eyes make mourning look so good, so right, that everyone agrees: this is how beauty should actually look.