I've had the same problem - rooted the mini Pro, then couldn't download the update and couldn't repair it using SEUS (saying I had latest version) or PC Companion (error with internet connection) and couldn't unroot it.
I've sorted it by using Omnius mentioned in another thread.
Register on the site and download the software and the ROM. I chose the U20i_WORLD_1_8_CDF1238_0199, which on a thread about changing the colour listed it as UK Black. It's a RAR with 2 files. I set the larger file to be Application file and the smaller one as the Customisation file.
Flashed it and it worked, back to stock without root. Took a while to boot from the SE logo, I thought I'd bricked it!
Downloaded the official OTA 1.2.A.1.174 and it installed. Going to test now, hoping that it may improve battery life, my only complaint with the phone.
Mine's a sim-free x10.
PS - It's still blue scheme, not black.