Excellent replies - I totally agree with the idea that you can't know what someone's response is going to be.
In this way I feel some aikido training is inappropriate if you say, 'well uke will do this' unless you know what to do when uke doesn't do that. However I do think you can train yourself to respond to someone changing their weight, posture, arms, once you have initiated your technique.
Also, I fully agree that there is a good chance of taking at least one blow.
I feel atemis are essential (it stops an opponent knowing exactly what you're doing for one), and should be practised. For this reason I would expect uke to deflect an atemi (otherwise aikido would be quite painful). If the atemi is not hard, it may not be neccessary for a 'good' 'block' as it is just for protection during training and not mainly for ukes benefit.
However in close situations, even a weak 'block' has benefits, because if you get used to doing them, as long as you are moving your body out of the way of an attack a weak 'block' allows you to get contact. (and you can do these very fast. - as mentioned before, you shouldn't expect to be just covering your face, anywhere that is exposed you should be prepared to deflect/block).
As far as seeing goes - I'm not really sure if it is the most important thing. I think I would rather cover my eyes with a 'block' and know I have contact with his limbs, than be able to see a punch coming straight for me.
On that note, has anyone ever tried Aikido in total darkness? Maybe its more realistic?