I guess so. But I imagine if I tried that shit at a TT game, it would result in a 20 minute argument with the opponent. They'd argue "...must immediately throw a second block." Must is non-arguable. Immediately is before doing anything else. I'm just saying, it's a gray area that could be argued either way.
@dragonloup I don't have BB2016 which is the most current rules, but if you google competition rule pack blood bowl there is a pdf of the CRP (the older but most recent set of rules BB2016 took the place of.) But nowhere in it does it mention being able to stab on second block other than in the description of Frenzy skill "Unless otherwise overridden, you must make a second block." So, Stab must override Frenzy.