black hair

Rule number one: Color does not lift color. No matter what kind of dye you put on your black-dyed hair, it won't budge.

You need a color stripper. I've used One n' Only Colorfix myself with great success. It's available at Sally's. It may take one or more rounds of Colorfix to remove all the black, and make sure to follow the instructions to the letter (especially the part that involves adding developer to tell if you've removed all of the color). This will likely leave you with orangey hair. You can then easily put brown dye over the orange.

Also, read the FAQ from the link posted here http://p080.ezboard.com/fhair18356frm17.showMessage?topicID=44.topic (I can't get the direct link to work because it's a web archive)

First, Colorfix is not a stripper...Secondly, If you look at customer reviews for Colorfix they say that it turns your hair orange(which, yes, you can redye brown) but even when they redyed it, it went back black after a few days..Its not supposed to damage your hair, so you can give it a try if your tight on cash. If not, I would suggest you go to a pro(they dont use Colorfix)--find one that uses a product called Rusk Elimin8 which is similar to ColorFix, but more succesful at removing color and it shouldnt damage your hair either. Both of these things will not give you a guaranteed pass back to the color your aiming for, so be aware of that...The other two options you have would be to have a stylists strip the color out for you which can be slightly damaging..Or you could have it bleached and recolored over which I would definantly NOT recommend. Anyway- good luck.