Natural indigo powder creates a nice muted blue color in cold process soaps. Indigo can be added directly to soap but many prefer adding it to the lye water to get a darker, richer blue. For best results, add about 1/2 tsp. of indigo per pound of soap to your lye water and then add the solution to your oils.
This particular kind of indigo is known as Qing Dai in Chinese medicine. It contains 2.2% blue dye and is a highly pigmented form of indigo.