Update:
Bought a new different model tube from LBS this morning (after the ride left of course) : same steps as before and no blow out this time.

It can't be a something in the tire even if somehow I missed it twice : it would hiss not blow up so that the whole hood can hear. I did look carefully the second and third time around at the rim tape and though I am not a specialist I saw no sharp edges etc. Besides why would it blow and not simply puncture/hiss?

It is either two pinches one after another (even though I was careful god damn it!) or bad batch (same operator but no blow out with the third tube I put today).

Bottom line: today is min 60 - max 73 degrees but I did not ride. I am bummed and and at work...