I know but think about it, Kakashi did have his Sharingan activated when he warned Naruto in that scan I showed? Hence he should have seen the same as you stated in the bold but somehow didn't. Naruto is a sensor(Sensing the Kages from miles away) so his blindspot doesn't matter in this case and we both know he can easily dodge someone teleporting to him from above with such speed. Especially if he reacts to the likes of Ay's full speed.
"Naruto had to be warned here to react to this attack
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He was surprised but still ended up evading. If someone intercepted the attack for him, are we then going to conclude that he couldn't evade because he was surprised and after all, someone reacted before he could as he wasn't shown to react?"
It's just like every Uchiha who attacks...If they can foresee one dodging/moving from their attack before the make it, then they shouldn't be surprised or even attempt it further but we've seen scenario where they end up being surprised or caught slipping back.
B was only surprised when Itachi was in Mid air. Itachi wasn't in control of his actions so when he said 'Behind you" the second time, if B wasn't fast enough, he won't have reacted like what's being claimed when Nagato warned him.
However he reacted fine and this shows that whether surprised or not, he would have reacted even while warned by Nagato had Naruto not been there with him..Not sure if you get me? Like one said, he had no intel on Itachi's capabilities and so did Itachi..Though he just felt the need to help them in a worst case scenario. Like how Edos did in the war.