Perhaps I was just a bit too general in my description of a block. I understand that the object of a block is not just to "stop" the attack. I've also studied Karate and Wing Chun Kung Fu and will agree that in the ideological sense, the block is meant to redirect the attack. However, the type of redirection that occurs with a Karate block is a little different than the type of redirection that occurs with an Aikido technique.
An example would be an outer forearm block (sorry, don't know the Japanese term). In Karate, the incoming punch would be redirected to your (if you are being attacked) side, away from your center. In Aikido, the Nage might perform a tenkan (turning / pivoting) movement and redirect the punch downward and behind the attacker, like in Tenkan Kotegaeshi (outer wrist lock / throw). The same could be said about an overhead block in Karate. The attack is redirected off your center, perhaps over the top of your head. In Aikido, a Shomenuchi (overhead strike) is not redirected over your head, but rather in the direction that the attack travels, downward.
I apologize if I was too general in my original post. Jim, I added the English language conversions just for you