It’s a historical term which has been around for a long time. Therefore it has a meaning, to the common man.
For example, it is a fact that Napoleon was 5’ 9” tall. However the media in his day slandered him and made fun of him being “super short”. If anyone nowadays is said to have a “Napoleon complex”, most people will picture a short, angry guy even though, IN FACT, Napoleon wasn’t short, nor angry.
In the same way, it is known in common opinion that the head of the jesuits was called the “black pope” by Protestants. Therefore, most people know this nickname and it’s part of common knowledge...even if not historically accurate.