My Sensei says it's not a block; you raise you arm and seek the minimum contact and position ideally necessary to have uke's "chop" slide along the external side of your raised arm harmlessy instead of cutting through and hitting your face.
This becomes crystal clear IMHO if you do/think the thing with bokken, both uke and tori, or (harder) only uke....
You won't block a cutting bokken (or worst, a sword) with your arm, do you
? And you won't block it with a bokken either, better have the attacker "sliding away" along your diagonally raised bokken and simply counterattack with a direct clean strike after uke's attack fails and breaks his posture due to the sliding.... a block permits him to mantain control.
Barehanded vs. bokken, you will enter directly, the "contact" is only of mental nature to find the correct timing.
I think with or without bokken is only a matter of forms, the principle is the same and perfectly logical.... a block violates the logic of aikido and kenjustsu IMHO, and is practiced only for didactical reasons for beginners.
Not even considering the fact that you may not want to block the strike of a 2 meters, 115 kg of muscles drunken brute... your arm will suffer severe damage, if even able to block altogheter.
The more i think about it, the more the block seems illogical to me from an aikido view