I do think that Naruto was 'losing his way' in the darkness and not a genjutsu, but you couldnt really be sure it wasnt an illusion trap. Unless ofcourse you read the manga and know what happens anyway.

This is my possible explanation on how it could have been a genjutsu.

Well basically wouldnt all ninja learn basic genjutsu during acadamy? Just because you dont see them learn or do a genjutsu it doesnt automatically mean they cant do it, no?
And surely you dont have to make eye contact with a person for an illusion to work, so not his sharingan for obvious reasons.

Even if it was only a weak illusion, that plus the darkness would be effective. All sasuke wants is to get as far away from Naruto as possible, so doing such a thing to slow Naruto down cant be ruled out really.
So seeing it in that way and not reading the manga to know for sure, it cant be seen as a ridiculous explanation, as some of you have strongly shown.

(before you 'omg its not genjutsu, dumbass!' people hang me, just consider it from an anime watcher only perpective. It doesnt seem so far fetched)