nicolas_noir & i5rovic.
The blue line is flare. To avoid it shoot away from the sun, but I've found I get similar flare on unexposed shots. If you don't like it, maybe "flock" the inside of your camera, which basically means painting it black.
Thanks for the answer. :)
It looks like flare but it is visible on every photo (away from sun or not). La Sardina is already black on the inside. :(