Your best bet for the perfect scene in the film to check would be after Jamie Lee Curtis stabs him off the edge of that ledge. When you see him all sprung out over that table, his coveralls look like they are dark grey. Another good spot to check out would be in the begining when he slices I beleive her name is Mrs. Loomis, Neck open after she breaks the window with that golf club. Theres a couple scenes right there were it shows him walking in the house and some of the lights from the cop cars beam on him.
Other than that I would say that the blue look comes from a combination of the grey and some of the light weathering on the coveralls.
--Harry
Thanks Harry,
I actually did post that pic
. .. just not here, I put it in the Screen Caps section.
Come to find out, it's not as good of a shot as the one I posted above . .. too many shadows.