going back to dark brown

I don't know how extreme the difference is between your two colors, but how about putting a clear gloss over the whole head just to even out texture and help hide the difference? Don't strip your hair when you've worked so hard to grow it out!

If the difference is too extreme, get a semi-permanent (deposit only) at Sally's like colorgloss and just put that on. But you might have to keep using it and it will change your natural hair slightly. But then when the dyed color grows completely out you could switch to gloss only until it is all gone.

So its halfway down your hair, and the top half if your natural dark brown which you want?  I'd just wait it out, its not much longer now.  If you start messing with it now you'll have to start over your growing out process to get your natural hair color back (because you probably won't have much luck just dying the previously dyed parts and getting them to match, so you'll be processing the new growth too, and even then it may come out two different tones.  Hair normally grows about a half inch a month, so you can calculate how long it will take to grow out if you want to keep the same length you have now.