Black tea is the most oxidized tea. The tea leaves are allowed to fully oxidize before they are heat-processed and dried. During oxidation, oxygen interacts with the tea plants cell walls to turn the leaves rich dark brown to black color. It comes from the Camellia sinensis plant and is often blended with other plants for different flavors, such as Earl Grey, English Breakfast or Chai. Black tea contains polyphenols that have antioxidant properties, flavonoids, that are said to combat inflammation and support healthy immune function. Studies have shown theaflavins found in black tea can reduce cholesterol and blood sugar levels. Black tea has caffeine.