It is without a doubt an unusual, but not unappealing, shade of blue. It is not black (though you may call it whatever you like).
I'm sure Samsung did this from a marketing point of view. Basically, so far in the smart phone world, we've had for the longest time now either black or white. And that's been it. When you think about it, after the Galaxy S II and the slew of competitor comparable smart phones that came out after it, they all look very similar. I can just see Samsung's marketing review as having said that they need to go with this new shade of blue because it will distinguish their new device from the sea of all the others. They want to set themselves apart in terms of performance and appearance (which is of course the way to do it).
However, honestly I'd just be happy with black. Always like black. But then, with cases, the colors are ultimately not going to matter that much anyway.