Saturated blues and purples are harder to look at than brown. Brown is a pretty dull colour, so it's easier on the eyes. The blues work in a game full of sprites, where you only have 2 dimensions and everything has clear outlines, but in a grand 3D game like Dark Souls, that much blue get be painful to look at pretty fast.
Not saying using blue wouldn't work at all in Dark Souls, it probably could, but personally I think it looks fine as it is.
Not saying using blue wouldn't work at all in Dark Souls, it probably could, but personally I think it looks fine as it is.