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 188.8.131.52. Guess what?! Same issue!
I am crushed