The black chairs were purchased by Joey in season 2. In episode 4.02, they were stolen (along with most of the furniture). Then in episode 4.07, Chandler bought new brown chairs (and other items for the apartment) because he was feeling guilty about stealing Joey's girlfriend (Kathy). Why didn't Chandler buy new black chairs? They looked so much nicer! Well, the black chairs made filming the show more difficult, because of the way they absorbed light.