Ted Newsom wrote:

Again, as far as I understand such things, there was no such animal as a "blue backing" matte process in the silent or early sound era. The concept was the same: photographing an element in front of a limbo screen, which then creates the element and a mask-- the "matte"-- to print into another shot. This could be done with either a limbo black background (as Bert Gordon did years later) or a white background. The Dunning process is pretty similar, using (whoa. Where's Jim Aupperle when I need him?) red light to create some sort of differentiation effect between subject and neutral background.



Ted, the Dunning process was an in-camera bi-pack traveling matte that did indeed use a blue backing, with the foreground elements (actors, models, etc.) lit with yellow/orange or red tinted lights. (I've read about a number of variations.)
A positve yellow/orange/red-dyed print of the background scene was run in front of the negative film, so that the  properly lit foreground image would be exposed over the background -- hopefully, with no print-through -- the blue background turning optically black.  

Though it used a blue backing, the Dunning process wouldn't work on color film. But blue became the color of choice in most color matte processes anyway, simply because there's not much blue in most skin-tones.
(Sodium-screen mattes are another bi-pack process, using color and high contrast B&W film in the same pass, giving you foreground plate and matte, to be combined with the background plate in an optical printer.

During KONG, I believe Linwood Dunn came up with a refinement of the Williams system, using the optical printer they developed to insure better-aligned  mattes with less edge-shrinking/expansion. Simple hand rotoscoping could handle any bleed-through areas (time & budget permitting), as long as the edges were clean.

And you could fiddle with it in post, rather than doing re-takes with principal actors until you got a good matte.

BTW, varitions on the Williams process via bi-pack camera were used well into the 80's. A lot of the traveling mattes in the 80's tv series V were done that way, in tandem with motion control.