Hi, we know that green tea is getting popular for its medicinal properties. Both green tea and black tea come from the tea plant called Camellia sinensis.
The difference of geen tea and black tea comes from the processing method of the tea leaves. You can get green tea just after the leaves are picked, steamed and dried immediately, where in case of black tea it under goes some natural enzyme action which changes the leaves to black.
Important difference in green and black tea is caffeine content, a cup of black tea contains 10-50mg of caffeine but 1 cup of green tea contains 30-50mg of caffeine.
You have asked about black tea so we move on that, the recommended intake of black tea per day is three cups. If you see the advantages, reduce athma problem, presence of tannins in black tea reduce diarrhea, research conducted by American Heart Association in New Orleans black tea reduce the risk for heart problems. Regular intake of black tea reduce cancerous growth in the breasts.
If you consume more than the recommended quantity, the excess caffeine cause mild headache, stress, sleeplessness, nausea, irritation, irregular heart rhythm etc.