Wildly enough, this is a burnout print.  Burnout prints actually "burn out" (remove) the cotton from the shirt, leaving only the synthetic fibers, generally polyester (in this case also rayon).  

Most blanks used for burnout designs will leave a white, or very lighter color, image.  

We've found both American Apparel and Next Level Apparel's grey Tri-Blend's to leave a nearly black image, which we think is pretty rad!  

Why is that you may ask?  The answer would lie in the shirts synthetic fibers (the only remaining fibers after the "burnout").  The construction of these shirts is 50% polyester, 25% cotton and 25% rayon.  Since we've "burned out" (removed) the cotton in this image area, only the polyester and rayon remain.  Since the image area is now nearly black, that leads us to the conclusion what the cotton in this shirt was obviously very, very light colored cotton.  We can also assume that these shirts have very, very dark polyester strands rather than white polyester like most 50/50 blanks.

That is today's Science 101.