The hex files your using have nothing to do with the black screen as long as they have peek/poke enabled. The problem lies in the usb device youre using to store your dev_flash. I reformatted mine from fat32 to fat and it worked but only after several bad attempts of just copying the files over. Its either that or youre failing to get complete rips of your dev_flash.
i've tried different USB storage, a external USB HDD, my PSP and a MP3 player. ive tried with them plugged in on startup and without. I just changed FTP clients and recopied the files over and same result. i have no idea.