A great tea master once told me that the best tea processing is the processing that cannot be tasted. In the case of charcoal-roasted teas, I would tend to agree. Some people might really like to taste the charcoal, but I like to taste a rounded, balanced, full-bodied tea with sweet notes that add to the tea, rather than a mouth full of charcoal. Just as when one grills meat over charcoal, the objective is to heighten the flavor of the meat, not to taste charcoal or, worse, lighter fluid.
Charcoal-roasted teas have a very distinctive character that is usually quickly recognizable. When the tea is both carefully roasted and brewed optimally, the roasting adds a rich carmelized sweet note that heightens the flavor of the tea and rounds out or balances other notes in the tea. When charcoal roasting (or brewing) is done carelessly, what is left is the flavor of the charcoal which overwhelms the taste of what otherwise might have been a marvelous tea.
Teas that lend themselves to charcoal roasting include Taiwanese Dong Dings, Wuyi varietals grown in China or Taiwan, and Ti Kuan Yin varietals grown in China or Taiwan. These teas are typically brewed in hotter water (190-200ºF), but I notice that if I brew them in slightly cooler water (175-185ºF), the sweeter notes become more dominant, the tea has a smoother mouthfeel, and the charcoal roasting is not as pronounced.