I had something like this happen when I upgraded my 3.41fw. Screen just went black and I wasn't sure if the upgrade had completed or not. Not the best feeling when you know you shouldn't switch anything off while the firmware is updating...
Anyway, I waited about half an hour and then I rebooted - everything was fine and the fw had upgraded.
My only suggestion is to see if the yellow light next to the usb sockets is flashing or is it steady? If it's flashing, the console is still doing something. If it's not, then it's probably finished upgrading and your screen has just got black.
Also, your controller is attached to the console with a usb cable yes?
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr