I tried to naturally dye cotton and silk fabric blue with black hollyhock flowers. I poured boiling water over the hollyhock flowers and let the solution steep over night. On the next day, the soaking water was purple. Then I put silk and cotton fabric scraps into the natural dye solution and let it soak for some hours. By the way, I didn’t use a mordant or modifier. After rinsing the scraps in clear water, the cotton turned a pretty blue, while the silk stayed light purple.