Want to dye my hair from black to dark brown, is this impossible?

To top it off my hair is very long, making this even more difficult. Is the only way to get my hair dark brown to use hair dye remover and then dye on top of that? When I dye my hair I only dye the roots, so some parts of the bottom have faded a little to a dark brown... does this mean the bottom parts of my hair are more "natural" and thus susceptible to hair dye? lol I'm sorry if this is a stupid question, just hoping there's something else I can try. Thanks!