Cover the hibiscus flowers with 1.5 quarts of boiling water. Slit the vanilla bean lengthwise and scrape out the seeds. Add the seeds and the vanilla bean to the hibiscus and steep for 20 minutes. After 16 minutes add the black tea, cover, and let steep for the remaining time.
Pass the tea through a fine strainer. Season to taste with lemon juice and the rock candy. The tea can be served while still hot or chilled.