i don't know i think he's been always that cheeky while facing certain opponents specially if they underestimated him
even so i wouldn't call it darkness he still has his gags and funny moments and he's still as light hearted as ever when it comes to dealing with his allies
EDIT : well look at how he dealt with monet
First… It is just my opinion.
I miss this Zoro moments
http://www.mangapanda.com/103-2070-19/one-piece/chapter-21.html
http://www.mangapanda.com/103-2073-4/one-piece/chapter-24.html
http://24.media.tumblr.com/tumblr_ll6x4iaxlG1qgboibo1_500.jpg
and so on
But once again… Just my opinion that he got too bad-ass, which makes him stoic in my eyes... Same as others dislike the overdone Sanji woman shit post TS.
And as I mentioned before... it is not just Zoro but some others as well. But well we have a long way to go, so maybe their will be moments like this once again