What's with the box?

I designed a t-shirt recently and while the graphic looks good, the issue of the print wasn't as I expected. It is just white text on black and the white isn't bright. I understand it is white on black but the white is now a dull grey. After one wash (yes following instructions) a grey box appears over my chest and torso area. It's a heat transfer decal. This is disappointing to me. Despite this, the shirt is comfortable and fits well. Overall, it's an okay shirt but I think the graphic quality could have been better. Now to see what happens to my navy blue shirt.