Ooooooooh! I never knew that!
another fact on Katana combat while we're at it is that Katana's had one edge thinner than the other. In fact the slicing edge would be much thinner and therefore more brittle while the blunt edge would be thicker providing support. This being the case, open combat mandated that the Katana never be used head on to block moves. Using the sword edge to block a move was considered poor swordsmanship. The idea in using a katana (and a wakizashi too I guess) was the evade-and-counter move.
There was however one situation in which the Katana could be used to block a head on attack. This move was based on the Katanas curved nature. When facing an attack from above (like a sword slice from above), the katana's convex curve could be used to deflect the attack. The curvature of the blade itself would ensure that the edge did not spoil from such an attack.