How can I get an effect like the blue in this image?
Here's the Photoshop (CS6) formula I used...
The overall work is split toning, but with some added pieces in to it. Not a perfect match for what you did, various subjects and surfaces will matter here, but here's the result I got to with a little work:
Play around a lot more with the settings layer order and use masking to get even closer to the output, especially on the skin.
You could try split toning (There are plenty of examples here)
This is a good article about the technique, and includes links to other sites which show describe the best approaches in the main photo-editing applications (Photoshop, Lightroom, Aperture, ...)
(instructions here for CS4)
Start with a fairly contrasty mono (or the whotes don't show through), this works well with non-destructive processes.
...and adjust to taste.
Edit:- here's the pic I was going to post. Could probably do with being a bit lighter in the mono conversion but hopefully it gives you the idea - full whites are white, blacks are black, midtones are tinted.