Hey, might not be an issue at all, but in the video, I saw that the two times you were able to get it to go blank you had your finger on or near the proximity sensor. I wonder if there is something that could be the trigger.

Just wondering if you can duplicated it or cause it only when you have the proximity sensor blocked, or if it truely is random.

You are right, it might has something to do with the proximity/light sensor.
On the other hand, this issue occurs even on incoming calls when the phone is placed on a table, screen up and nothing around the sensor.
Today, I visited my local carrier shop and left the phone for repair again. They told me, since this is a second claim they'll run it through the process faster, so I expect to get it back next week.
Will post an update.

UPDATE: My phone was replaced. Yesterday I got a brand new unit, yeah! It was stock wp 7.0. I plugged in my sim and used the phone the whole day, no sign of blank screen issues. Last nigh I installed all the updates (total 5 I think) up to OS 7.10.8107.79, FW 1.0.16.10. Guess what?! Same issue!
I am crushed