What exactly do we officially know about the black shirt? All we know about the black shirt is from the Entertainment Weekly Magazine article that said “Kirk spends much of Star Trek dressed in respectable black, space-cadet colors in Abrams’ Trekverse" and from Orci who said the shirt is black "for a reason".
There is not enough information to convince me that Kirk is a simple underclassman cadet when he takes over command of the Enterprise. I think he may have been in the Academy for some reason and the black may signify that, but I think perhaps he is an officer at the time.
I can't remember if it was hat article, or another - I just remember somewhere that black was cadet colours.
Though I'm not thinking he's on the same level Wesley Crusher or Nog was, officer in training, but more what Saavik was in Khan. A graduate in command training. It gives the story enough logic for Kirk taking over - though from the trailer details it seems as if Kirks came back from the ice planet after a Spock pep talk. I'd thought before that, and there might still be - I'll find out in May - that Pike gave the nod to Kirk which made him take command.