I always figured that the blue liquid was just because too many people would be grossed out by seeing something that actually looked like blood. Lots of people freak out just seeing blood from a scratch, not to mention the "unmentionable" type.

Come to think of it I read somewhere (BBC something on the "disgust" reaction) that blue is a colour people connect with clean, sterile, laboratory, non-biological, so they don't react negatively to it.