The art has matured too, it has always been beautiful, but her drawing skills have improved a crazy amount. I was just looking back at that scene where Mey-Rin tried to snipe Ciel, then there is the panel of Sebastian deflecting the bullet, and that particular panel, I realized that wow, if I compared this to the first volume of the manga, it would be hard to believe some parts are the same author. The texture especially has changed, the one now has so much detail!
Then: http://www.mangago.zone/read-manga/kuroshitsuji/an/kuroshitsuji-chapter-3.html/16/
Now: http://www.mangago.zone/read-manga/kuroshitsuji/an/mpn_chapter-2221350/pg-15/

Just casually naked xD