If they are already in blockstun, then they can probably still block the move that has a block area of 0.  If you want to make a move unblockable while opponent is in blockstun, then you'll need to code that in.

It's not too hard actually (create a bool "Hit.unblockable", and during the Hit checks under // Block, just check the !hit.unblockable), but keep in mind that you're potentially creating unintended behaviour (unblockable situations [block strings], which should always be avoided in games).  Put the move on a timer, require meter or some other way to balance it and you should be fine.