Update: I've just spent a bit of time polishing this up, there were a few problems with it that I hadn't noticed before because of crappy uncalibrated monitor. Oh and I fixed the awful title and replaced it with a much simpler one.
Thanks very much! To be honest, when I made this piece, I did it in a very rushed way, which meant a lot of the picture wasn't planned. So that blue line probably just appeared when I was scribbling with the brush!