The difference between black tea and other types of tea—like white and green—is that black is more oxidized, a result of the leaves having been aged longer.
In the past, black tea has been shown to improve mental alertness, prevent Parkinson's disease, and relieve hardening arteries.
The researchers conducted an experiment involving 111 men and women who showed signs of prehypertension. Participants were asked to drink three cups of black tea a day and had their blood pressure monitored three times in the span of six months.
The results of the study showed that drinking three cups of black tea a day altered blood pressure variation by about 10%. The effects of drinking black tea were noticeable after the first day and continued over the course of the six months. The researchers believe there is an ingredient in black tea—not caffeine—that accounts for this positive effect.