Is there an opposite form of vitiligo? is there a form of vitiligo in which patches of the skin ever become darker?

Hyperpigmentation: Dark skin patches are a sign of skin inflammation and usually occur in darker skinned individuals. These can results from several forms of dermatitis that inflame the skin. Treating the underlying cause can eventually clear the dark areas.

Not really: There are conditions where women can develop darkened /discoloration of the skin (usually the face) called melasma. This is generally caused by birth control pills, or pregnancy. But patches of skin wtih vitiligo can't become darker, b/c in vitiligo, the pigment producing cells called melanocytes have lost their ability to produce pigment.