This is a bit of a sticky problem, which I admit I haven't come across before. If I were you though, I'd try to select the parts of the shirt you want to change then hit ctrl+j to make it it's own layer. Image->adjustments->black&white and put all the sliders at black. If that isn't dark enough, follow it up with image->adjustments->brightness/contrast and pull the brightness slider down until it works (repeat this step as necessary).
this'll probably require some tweaking, but it should at least get you close.