A black denim shirt would be pretty fantastic. Though with the black denim, I'd definitely prefer either a smoke resin button as opposed to the tortoise resin. While the tortoise works great with the blue, I think it would look a bit off with the black denim. Also, I'm a big fan of the pockets used on the denim work shirt: I've always preferred these kinds of pockets over the flap pockets. The point collar is a nice change of pace as well.