Considering it destroys the sword and kills the wielder, it seems more like a weapon created for a specific purpose and to create the power requires a great sacrifice. That, I don't think, makes it too munchkin-like.
The small black holes it makes, how about that the black hole allows the blade to go deeper, but if it goes too deep the black hole is such that it curves space so that the sword goes through the black hole and comes out into the wielder in the same place he is stabbing the victim.
So I stab Joe Evil NPC in the shoulder and continue to push through, the push will eventually come out my own shoulder. Maybe the blade will _never_ go through another person, the black hole will stop it from coming through the otherside and instead put it through the wielder. The wielder would have to train himself not to stab too far else get himself. So each hit would require much more concentration and finesse.
---That is where this is familiar.
1) Spiderman had an enemy that attacked through these little black hole portals. In this, little black holes would appear all over the place surrounding each of them. You had to choose the correct black hole to punch through in order for it to come out the correct hole to hit the opposite person.
2) Vampire Hunter D had an enemy that, if stabbed, would not penetrate him, but shift to stab D instead.