I have dabbled in dying natural materials for 15 years or so. Black is the one color I won't do. I only know how to use hot dye methods. Rit or Venyard. Black takes more heat, time, dye and vinegar. All can be bad for the material.
Some may suggest dye it orange first. I haven't tried this.
Black is just too hard for me to achieve.

Sorry I couldn't be of help.

The guys that use cold water methods may have a much better handle on getting black than I do.

Bob