We know that "block any attack" is hyperbole. It's a very strong shield, resistant to elemental attacks to give it a boost. However, Susano'o has limits. And anyway, Itachi lacks the stamina to deal with Naruto for long periods of time, while Naruto has nearly endless chakra. Naruto has Bijuu Mode speed and Sage Mode reflexes, and can enter BSM almost instantly.
Itachi is very powerful, and he'd be a challenge for Naruto... but Naruto's raw power makes it such a safe and forgiving match-up for him, while Itachi needs to make every move count.
I don't mean this as an insult to Itachi or his fans; Itachi is great, and exemplary as both an Uchiha and a ninja. However, Naruto is on another level now. Their powers were roughly equal while Naruto only had KCM, perhaps leaning towards Itachi. This balance shifted when Naruto gained Bijuu Mode, and Naruto had a clear edge. At this point, BSM is just too overwhelming when you consider that Kabuto's Sage Mode enhanced his reflexes to the point where he could avoid Amaterasu; Naruto has that, on top of being able to use BM to sense when an attack is coming, and far more raw speed.
