This shirt is very similar to Dean’s S8 Army Green Shirt or Dean’s Dark Blue/Black Shirt, but it appears to be black and also has black buttons. It seems to be made of a heavier cotton than Dean’s Dark Blue/Black Shirt, so I’m counting it as a different shirt. Hopefully I’m not wrong!
Dean wears this shirt in Captives (9x14), when he and Sam prepare to hunt the ghost that suddenly appears in the Bunker. He wears it until he changes into a suit in order to investigate the storage facilities.
In Black (10x01), Dean wears this shirt on the second day we spend with him and Crowley. We first see it in the scene when Crowley tells Dean that he’s been the one sending the demons after him. Dean wears it until the end of the episode.
In There’s No Place Like Home (10x11), Dean wears this shirt at the beginning of the episode when he eats egg white omelettes with Sam until the leave the Bunker to go try to find Charlie. He briefly wears a suit, and then changes back into this shirt while he and Sam drive to the next location, where they once again put on suits.
In About a Boy (10x12), this is one of three shirts that we see Dean wearing during the research-montage at the beginning of the episode.
In Love Hurts (11x13), Dean wears this shirt at the beginning of the episode, when he is still hungover from Valentine’s Day and in search of food and coffee.
In Somewhere Between Heaven and Hell (12x15), this might be the shirt that Dean is wearing at the beginning of the episode when he and Sam get back from two solid weeks of hunting and Dean is very dirty.
In Ladies Drink Free (12x16), this might be the shirt that Dean is wearing in the hotel lobby, when they catch up with Claire, before Mick calls it an early night.