Try this (from http://www.phytochemicals.info/research/tea-antioxidant.php), though i can't tell if that is what makes the tea black or something else, but usualy oxidation does that:

The health benefits of tea are mainly attributed to its polyphenols such as catechins, theaflavins and thearubigins. Catchins are flavanol monomers. The most important catechins are epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate . Fresh tea leaves contain naturally polyphenol oxidase enzymes. When the tea leaves are crushed these polyphenol oxidase enzymes are released and start to polymerize the phenols into flavanol dimers and the polymers theaflavins and thearubigins. Black tea leaves have undergone this oxidation process (also called fermentation although no real fermentation with bacteria is involved) and contain higher levels of theaflavins and thearubigins and lower levels of catechins. Green tea is first steamed to destroy the polyphenol oxydases. As a result green tea contains still high levels of catechins and lower levels of theaflavins and thearubigins. Green and black tea also contain the flavonols kaempferol, quercetin and myricitin.