When I speak on natural medications (herbs, vitamins, and supplements), I tell folks that my favorite natural medicines website is the Natural Medicines Comprehensive Database which has new information about Hibiscus (Hibiscus sabdariffa). NMCD says, “Hibiscus is getting more attention as a potential treatment for hypertension. New clinical research shows that drinking a specific hibiscus tea (Celestial Seasonings) three times daily for 6 weeks significantly lowers blood pressure by about 7 mmHg in patients with pre-hypertension or mild hypertension.”
According to NMCD, “This is promising, but preliminary.”
Also,  I suspect most people could not (and would not) be compliant with drinking the tea three times a day every single day for years at a time.
So, the Database encourages us physicians to “explain to patients that drinking hibiscus tea might help, but it’s no substitute for conventional treatments that are proven to improve cardiovascular outcomes.”