Hi just to be clear, I downloaded a torrent of blank board serializer and made it into a bootable dmg on a usb drive, and then after clicking "option" on start-up I also got a black screen with a white underscore at the top left. Tried rebooting a couple of times, waited, and it went nowhere. Running a MBP 2.4GHZ Intel Core 2 Duo with a replaced motherboard..
Anyway,maybe this was a bum copy of BBS, that's why I asked for a copy (didn't get it). Truth is, after plugging my login/pass in the iCloud System Prefs window, over and over again, magically it started working. I still don't have a serial number, at least according to "About this Mac." Dunno if this helps.