I've dyed my hair black and I naturally have brown hair, how can I get back to brown hair without messing up my hair color with the black dye.

Black hair is the most difficult to lighten. And if you apply a permanent color on black hair, it will result in red tones. If you want to from black to brown, minus the red tones, you will have to pre-lighten her hair with bleach, then apply your desired color.