Both colors have the same specifications, assuming you are talking about 5th Generation iPod Touch. As for aesthetics I think the Black is better because the face plate matches the color. Where the Blue has a white faceplate.
Black looks extremely clean and everything is black. When watching movies and using apps that haven't been optimised for the larger screen, it would look better. It will also have a higher resale value than a blue one if you decide to sell it later on. Downside, shows fingerprints.
White/Blue is more fun and exciting, the white bezel won't show fingerprints but the white bezel makes the ipod look a bit messier because you can see the camera and the black outline of the screen.
I'm buying the black one because I have a white iPod Touch 4g and want to switch it up a bit. It really is up to personal preference though.