All factory books and guides, even the engineering drawings, are representative of what was supposed to happen, not what always happened. The black carpeting only myth has been circulating for years, yet I just found another 69 B body 4 speed survivor this summer with color keyed blue carpet-that makes 5 I can document. Had this same conversation with Frank Badalson and Dave Wise this year at Carlisle and they too have several documented. I don't think it was real common but it did happen.
It is also very possible this had to do with build dates!!

If you can't prove otherwise, put in black carpet to be on the safe side.