Sorry for bumping this thread, but I'm having the same issue as the OP was. All cia files updated successfully, shut it down after it stuck at the "reboot in 10 seconds" screen. Restarting without the SD card in, even in recovery mode, stays at a black screen with the power light on.
This is on an O3DS XL, downgrading from 9.8 to 9.2. I downgraded my own N3DS XL yesterday by the same method with no problems. Is there any fix for this or have I bricked it? If so, what the hell did I actually do wrong?