As long as the color you want to dye over the Manic Panic is darker, you should be fine. The Manic Panic is semi-permanent, meaning that it won't change the color of your hair underneath (provided that you didn't lighten it chemically, although I'm sure you probably did given that you wouldn't have very blue hair if you just dyed over medium brown; it would be more of a highlighting effect). However, you should take care of your hair and not dye it too often without conditioning it and taking care of it with sulphate-free conditioner and the like. I'm no expert, I've just dyed my hair quite a few times on my own, and I would advise you to take my advice lightly. I have dyed over semi-permanent dye and had no problem with coverage, but everyone's hair is different.
The color you dye over it, i.e. the brown (like what kind of brown? Chestnut brown? Mousy brown?) should cover well so long as it is darker than the blue. I don't know what color the blue wound up being but if it is faded to the point where it looks washed out, I would think that dyeing over it would be okay. As I keep reiterating:
1. So long as the shade you want to dye over your hair is darker than your current shade, it should be fine
2. Take my advice for what it is - what I have experienced myself - and get advice from a professional if you are that concerned about the results.
Personally, I wouldn't care as I don't want my natural hair color back anyway.