It was when Kakashi, Naruto, Sakura and Chiyo encountered and fought that Itachi clone. But I can't give you the exact chapter number, sorry.
And Kakashi might not be screwed. If only the eye with the MS goes blind he always has his normal one. But it would weaken him considerably.
Itachi on the other hand is screwed. Unless the MS makes you only half-blind.
So you can see if the Sharingan is active and be blind if es not or vice versa.
Sasuke would have to gain Ms before it will make him blind in the future, don't you think?
Because the normal Sharingan doesn't make you blind.
The talk about the eyesight between Itachi and Kakashi makes that clear.
Kakashi had his Sharingan for years, so he would've realised his decreasing eyesight sooner and I think he wouldn't have made a comment on it. And even if he would, Itachi wouldn't have been surprised. He knew that Kakshi had Sharingan and learn about that, so no need to be shocked.
So the one and only explanation is that the Ms makes you blind. That's why Itachi was shocked about the comment, since he didn't believe Kakashi could gain it (he said it almost himself).
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"