I bought Black because the Blue is such an ugly shade. That's just my opinion so it's much better if you actually take a good look at both at the store or some Nintendo event. Due to the system's finish, the color also looks different under certain light, so you'll need to have a good feel for it in your hands.
Aqua blue for me, it reminded me of a classic 50's car when I first saw it in person. Black is nice, but so is your TV, microwave, cellphone, coffeemaker, toaster, and everything you own that is black. --- 2D image of a 3D post.