Research has confirmed that both black and green tea are capable of reversing endothelial dysfunction, which is a key factor in the development of cardiovascular diseases.
Endothelial dysfunction is a systemic pathological state of the endothelium (the inner lining of blood vessels) and can broadly be defined as an imbalance between vasodilating and vasoconstricting substances produced by (or acting on) the endothelium.
Tea is an excellent source of antioxidant flavonoids such as catechins, which is the active constituent thought to be responsible for the endothelial and cardiovascular related benefits associated with tea consumption.
Black vs Green Tea – Equal Efficiency For Improving Endothelial Function
A study from 2008, published in The British Journal Of Nutrition compared the efficacy of black and green tea with regard to the amelioration of endothelial function.
The number of catechins in non-fermented tea (green tea) is typically higher than that in fermented tea (black tea).
Flow-mediated dilation (FMD) and nitro-mediated dilation (NMD) were assessed by ultrasound in twenty-one healthy women before and 2 h after consumption of green and black tea (2 h of FMD and NMD), in comparison with water (control).
In human subjects, ingestion of green and black tea led to significant increases in flow-mediated dilation(FMD). 
Flow-mediated dilatation (FMD) of the brachial artery is related to coronary endothelial function and it is an independent predictor of cardiovascular risk.
The increase in FMD was not significantly different between the two tea preparations.
The study concluded that both green and black tea are equally effective in improving endothelial function. 
Short and long-term black tea consumption reverses endothelial dysfunction in patients with coronary artery disease
Endothelial dysfunction in atherosclerosis is associated with increased oxidative stress and may be reversed by antioxidant treatment. 
A randomized, placebo-controlled, crossover study from 2001 tested the hypothesis that tea consumption would improve endothelial function in patients with coronary artery disease.
The study randomized 66 patients with proven coronary artery disease to consume black tea and water in a crossover design.
Both short and long-term tea consumption improved endothelium-dependent flow-mediated dilation of the brachial artery, whereas consumption of water had no effect.
The study concluded that short and long-term black tea consumption reverses endothelial vasomotor dysfunction in patients with coronary artery disease.
This finding may partly explain the association between tea intake and decreased cardiovascular disease events.
Green tea reverses endothelial dysfunction in healthy smokers
A study from 2004 investigated whether green tea consumption reverses endothelial dysfunction in association with improvement of oxidative stress in healthy smokers.
Smoking is a well-recognised risk factor for cardiovascular diseases. Free radicals in cigarette smoke are responsible for endothelial dysfunction, leading to atherosclerosis.
Oxidative stress is well known to participate in the pathogenesis of many cardiovascular diseases.
Green tea contained very large amounts of catechins, including epigallocatechin gallate, which has potent antioxidant effects.
The content of epigallocatechin gallate in green tea was 10-fold higher than that in black tea. Unlike black tea, green tea also contained ascorbic acid, which has antioxidant activity.
Green tea consumption significantly decreased the urinary concentration of 8-iso-PGF2α, which is a marker for oxidative stress.
The results of the study suggest that green tea consumption reverses endothelial dysfunction in healthy smokers, possibly through its antioxidant effect.
Earlier studies have demonstrated that endothelial dysfunction is important in the pathogenesis and clinical manifestation of cardiovascular disease. Thus, green tea consumption may be beneficial for the prevention and treatment of atherosclerotic vascular disease.
Further prospective, randomized studies of green tea consumption are necessary to examine whether green tea consumption reduces the risk of cardiovascular events and mortality. 
 The efficacy of black tea in ameliorating endothelial function is equivalent to that of green tea.
 The acute effect of green tea consumption on endothelial function in healthy individuals.
 Short- and long-term black tea consumption reverses endothelial dysfunction in patients with coronary artery disease.
 Green tea reverses endothelial dysfunction in healthy smokers
The information in this article has not been evaluated by the FDA and should not be used to diagnose, cure or treat any disease, implied or otherwise.