Help With External Editing

Hello all, so I am very new to blender and I am still learning Photoshop. I am texturing a gun for a game and I am trying to use external editing with blender and Photoshop, so when I get the image in to Photoshop and do what I want with it, I then apply it in blender and there is a dark rectangle around what I changed. I cannot figure out why blender (or maybe Photoshop) is doing this, any advice/fixes? A hasty reply would be great as we are on a tight deadline.
Any help is appreciated.
Thank you,
Here is the rectangle: