I know I said it already in my first post, I just want to make sure that you didn't key the bones used for blocking in your attack animation, and vice versa.
It's actually ok if you key them all in one of your animations, but then the order in which you apply the animations matters . For instance lets say your attack animation affects both right and left hands, but your block animation only affects left hand. You would apply the attack animation to the skeleton first, and then apply the block animation which would change the left hand bones in order to combine them.