The researchers asked healthy women volunteers to drink either freshly brewed 500 ml of black tea, black tea with (skimmed) milk, or boiled water as control. They found that while drinking tea (without milk) significantly increased the ability of the artery to relax and expand to accommodate increased blood flow compared with drinking water, the addition of milk completely prevents the biological effect. Researchers from the Charité Hospital, Berlin have found that the beneficial effects of tea are greatly reduced upon adding milk. It appears that proteins in milk bind to some of the substances present in tea, leading to a vast reduction of their beneficial effects.
Black tea is also responsible for the reduction of free radicals from the body, it is used by the people who want to loose weight.
I usually add honey and a bit of milk. Though, only with certain tea sorts. But I suppose I will stop use milk (not that it matters that much).