I think it's because the update you used didn't update the stock recovery. So the stock recovery still has the old drivers. I am not 100% sure though. I haven't rooted this phone in a long time now so its hard to remember the common problems people were facing. If you are planning on rooting the newest Xionia has both drivers and won't give you the blank screen.
PERFECT TIMING!!! I was just looking for a solution for this..I will give it a go tommorrow..THANK YOU..now i can get some sleep..