这里不是定语从句,这里是一个固定的句型结构“It is······that······”这是一个强调句,强调的部分就是第一个省略号表示的成分,任何一个句子都可以变换成强调句,你所列举的句子就是强调“she took off her dark glasses".这个句子还原就是：“I didn't realize she was Mary until she took off her dark glasses."因为强调句中对until 有规定：否定词要前移.
当然如果你还想问我关于定语从句和状语从句的问题也可以滴~·