Wow... I was getting all excited about something new to make tea out of in my garden. But here's what wiki says about it (article on hydrangeas):
"Hydrangeas are moderately toxic if eaten, with all parts of the plant containing cyanogenic glycosides. Hydrangea paniculata is reportedly sometimes smoked as an intoxicant, despite the danger of illness and/or death due to the cyanide.
In Japan, ama-cha, meaning sweet tea, is another tisane made from Hydrangea serrata, whose leaves contain a substance that develops a sweet taste (phyllodulcin). For the fullest taste, fresh leaves are crumpled, steamed, and dried, yielding dark brown tea leaves. Ama-cha is mainly used for kan-butsu-e (the Buddha bathing ceremony) on April 8 every yearâ€”the day thought to be Buddha's birthday in Japan. Ama-cha is poured over a statue of Buddha in the ceremony and served to people in attendance. A legend has it that on the day Buddha was born, nine dragons poured Amrita over him; ama-cha is substituted for Amrita in Japan.
In Korean tea, Hydrangea serrata (hangul:ì‚°ìˆ˜êµ hanja:å±±æ°´èŠ) is used for a tisane called sugukcha (ìˆ˜êµì°¨) or ilsulcha (ì´ìŠ¬ì°¨)."
So it sounds like the tea hydrangea is a different species than the ones we have here.