it´s hard to explain it technically for a digital environment, seeing as it origins from film photography. It´s basically about intentionally processing one type of film with chemicals you would normaly use for another type of film. this results in reversals of colors. film photographs tended to become more contrasty, get some kind of orangish / yellowish look to them and have abit more grain.
nowadays in the digital environment, most newbies associate it with "vintage look". it´s basically when you play with the levels in order to give the photo a certain colored touch. like with the blue you just mentioned. just google "cross processing"
To be fair, there is not a single solution that works for every photo. You have to fit your settings to each individual shot, and find out what looks best. if you master this, you should come up with something similar to ibarras pp. I linked you a video below, you can see that he´s doing alot more than just color tweaking.
http://www.youtube.com/watch?v=79f0es88PEAAlso a pretty good vintage look tutorial:
http://veerle-v2.duoh.com/blog/comments/photoshop%5Fvintage%5Feffect/