Sorry, I just checked again and the screen in my 3DS stays white. I just remembered it wrong.My USA console is on 9.2. Bottom screen transitions to white during the initial Spider payload that cleans up the bottom screen static during the exploit. Screen remains white after that. It could be FW specific even. Very odd you have it remain black while mine remains white.
I know of another user on USA fw that has his bottom screen remain white I believe.