Rehmannia is the herb to use for cleansing the blood of impurities. In this way it helps support the kidneys and liver especially.
An interesting thing about Rehmannia that is different from many other herbs is that it needs to be prepared ahead of time. The traditional Asian preparation is to cook the roots in yellow wine and black bean. Yellow wine is an alcoholic beverage made from rice and millet, and black bean is a type of soy bean.
This process makes the roots safe to consume (the raw root is toxic), and it also means they can be consumed whole, as they are, if desired.