Here are some dye samples I’ve made using black beans.
For column A, I soaked the beans 24 hours. It produced a light purple on silk, and light purple-blue on cotton. The colors aren’t lightfast.
For column B, I soaked the beans for 48 hours. It produced a light sage on silk, and light grey-blue on cotton. The colors aren’t lightfast.
For column C, I’ve used the same dye solution as at A but mordanted the fabric with iron liquor. It gave a dark blue on silk, and a bit lighter blue on cotton. After some weeks exposure to the sun, the colors seem to be lightfast.
I let all the fabric soak for about the same amount of time.
I’ve posted a tutorial how to dye cotton blue with black beans and iron mordant.