I'd actually like for this to be a power weapon. We don't have one in the melee category and maybe it's time to change this. We don't have a real "don't fuck with me" weapon; katanas and fireaxes can be engaged with bare fists, and it's even preferable to do so - considering blocking completely negates all damage and fists are the fastest melee "weapon" in the game. I'd be in favor of a pocket insta-killer any time, even if it's a single-use item (which it probably should be considering the potential damage it can cause).
Since any melee weapon can be blocked with the fists without the fighters taking damage, we're assuming the block action is not actually the fighter taking a hit and soaking the damage but rather redirecting it so they aren't hit. That's how they're surviving all these katana strikes, right? Let's assume this is the case with the shocker too. The fighter deflects the hit by concentrating on the opponent's hands rather than the weapon (even if we don't see that from the sprite) and that's how they stay on their feet. If that's the case there's no reason for the shocker to stun the player when they block. I'd say have the item be single-use in a way as to allow it to only shock once, after which it'll break, but have it be able to take multiple "blocks" as it were.
This would give the user multiple chances to effectively utilize the weapon while not making it massively difficult to deal with.
EDIT: I should clarify that I have a fast weapon in mind, as I use "shocker" in my post rather than "shock baton". It should be relatively fast to swing, that's why players using it get multiple chances to stun their opponent (getting hasty and "wasting" the charge on a block is not exactly the definition of "fun"). Essentially, you're setting the player up for an easy stun; the opponent isn't supposed to soak up the entirety of the weapon's durability without getting hit once (although this can be made possible if weapon balance is concerned). It's a power weapon after all.