Monochrome
Posts: 49
Location: Los Angeles, CA
Current Hair: Ruby red
Just be careful and do a strand test anyway with the Color Zap. It uses a little bleach, but has other caustic chemicals that can still damage hair - especially if the black you used was permanent dye.
If the black hair dye you have now is semi-perm or an organic based dye (like Aveda), then I'd do a strand test and it should work in one or two apps. However, if the black you have in now is permanent, I'd consider gradually lightening your hair to brown, waiting awhile, then a lighter brown, wait a little longer, then considering going to blonde from there. I know you have to be really patient with this method, but with the gradual change, your hair will definitely thank you. Permanently dyed black hair that's been dyed many times probably wouldn't withstand the Color Zap and break off.