I’ve always been curious / puzzled about how they could ever manage blue screens before computers:

In the special effects department you can easily use special filters to form two mattes from the shot of the actress. One shows the actress’s silhouette in black, and the other is the reverse.

These mattes are easy to create because the bright blue color, when run through a red filter, turns black. By using high-contrast black-and-white film to create the mattes, you can create the silhouettes. So now you have four pieces of film: the two originals and the two mattes. By combining these pieces of film in layers you can create the final piece of film for the shot.