I do have a 'New Camaro' brochure on the Camaro which would have come out ahead of the line. None of the artist renditions in the brochure, even the lighter ones has the blacked out rockers, however... in the text, buried deep late in the publication, the very last sentence of the last paragraph actually, states, "Also, depending on exterior color, body sill is black below side molding when Rally Sport is specified" The bow-tie and 350 fender emblem also are missing throughout the entire magazine.
I had my car blasted prior to digging down into the colors. Under my door sill plate the color is definitely the original Sierra Fawn as I had that taped off prior to blasting to keep a reference color. I did not blast the fenders yet and was able to sand down where the bumble bee stripe was and found black. I will look for the sill paint here soon.
Mine was a NOV '66 production.
In the picture above, that is a snapshot of the first Camaro commercial done, a 2 minute commercial on Youtube and the sill paint on that car is definitely not blacked out. Solid shots of the car throughout the commercial.
My suspicions are that it truly was an after thought that hit right when the cars met the dealers for the first time and then cars were then shot from then on.