If there was any filter present in the original filming, it's very subtle--the vast majority of the blueness in the original trilogy is due to the transfer job done by the company Lucas hired for the 2004 DVD version (needless to say, it wasn't ILM). Attached here for our edumacation are grabs from ESB and ROTJ from the GL Original Unaltered Trilogy version that was included as a bonus with the 2004 release alongside the blueified 2004 even specialer edition. These disks came in the very same package--just took them earlier today. As I understand it, Adywan's color correction work is done in order to get the best of both worlds: proper original colors with the nicer detail of the cleaned up 2004 transfer. I find the blue pretty objectionable myself, though I can see how some people would like it as being atmospheric in certain scenes or just generally in an old timey "silver screen" sort of way.
[attachment deleted by Tolwyn]