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.
(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.
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, ...)