Okay, I can almost guarantee that that was a glitch. As shown in the previous replay, behemoth blade can't hit thru obstruct. As shown by this replay taunt makes it struggle too. So, if it showed the animation and didn't fail, it must have been a glitch. If you dont believe me thats okay, but be aware that there are many glitches in showdown, as it is still beta, and personally I have experienced glitches too. Sorry I can't give a clearer answer but there is almost no way that it could hit thru it (unless somehow you had unseen fist on it) The only other possible option is that you misread it and it did fail. Again, sorry I can't give a clear answer, but it is highly likely it was a glitch.
EDIT:So you know how in the replay taunt hit thru obsrtuct?Well, turns out that's a glitch too, leading me to belive that obstructs coding may have not been done correctly