There are a few differences between green and black tea; the major one is that black tea requires a lengthier production process.
While tea leaves are heated almost immediately in the process of making green tea, the leaves for black tea are first dried in the sun.
The withered leaves are then rolled to break their cells, which causes the leaves to oxidize quicker when they are (next) exposed to oxygen.
This exposure takes place in hot rooms to accelerate the oxidation of the leaves, and it turns them a dark red/brown color. At this point, the tea is heated to stop the oxidation.
Black tea has a host of research behind it, and systematic reviews show that it can reduce blood pressure, lower type 2 diabetes risk, and improve the cholesterol profile