anything or anybody that was dark could not be called an object of beauty in that "old age." The speaker informs us that such thinking will no longer be valid any longer. Using personification of abstract ideas, the speaker states that Beauty is the parent or mother of Black. And now Black has inherited all of the features or qualities of Beauty. In other words, from now on, whenever people think about someone who is beautiful, they will immediately think of someone who has dark features (is "black").
In the last line of the first quatrain and continuing throughout the second quatrain, the speaker continues his idea of the personification of Beauty. In these lines the speaker suggests that all fair (light-featured) women who in the past were called the children or heirs of Beauty are illegitimate or "bastard" children who bring "shame" to the name of Beauty. The speaker explains that these fair women are not really beautiful at all because they have to apply make-up or cosmetics over their ugly or unattractive features ("the foul") to makes themselves appear beautiful ("fairing"). The speaker concludes this part of the poem by stating that one of two situations must be true. Either (1) Beauty has been "profaned" and "slandered" -- in other words, people have lied about her -- because these fair or blond children are not really her children, or (2) if they really are her illegitimate children, then Beauty should live "in disgrace." She should be ashamed at having such ugly children.