Where did this whole notion of the "Men in Black" come from? Well, the answer to that question is, according to journalist John C. Sherwood, a fascinating story of deception and social engineering, or cultural engineering, perpetrated by a man named Gray Barker.
Gray Barker wrote about UFOs and paranormal phenomena in the Fifties and Sixties. In 1956 he published a book called They Knew Too Much About Flying Saucers, which introduced the Men in Black into the folklore of UFOs. Somehow, this concept of shadowy, mysterious agents in black suits, white dress shirts, pork-pie hats, and skinny ties, tooling around the countryside in black Lincoln Continental convertibles (with those cool suicide doors!) to intimidate witnesses to UFOs and Mothmen sightings just caught fire with the public imagination.
A year or so after the first MIB movie hit the theaters, a prominent journalist by the name of John C. Sherwood wrote a piece for Skeptical Inquirer magazine confessing his own role in the wholesale invention of the Men in Black. He was only seventeen when he got involved with Gray Barker's myth-making exercise, writing under the pseudonym of Dr. Richard H. Pratt. Here's the link:
Gray Barker: My Friend the Myth Maker
Somewhere I read a related article, it may have been by Sherwood, also, in which the author describes how Barker hired actors to play the role of Men in Black and actually go intimidate (purported) witnesses to strange phenomena, such as UFO and the Mothman. He'd get them dressed up in the classic MIB apparel, put 'em in a black Lincoln, and send them off to hassle the witness, or witnesses. They'd roll into some little town in the boondocks where the witness lived, flash phony credentials, and drop dark hints and threats designed to scare the witness into clamming up. People would talk about it, of course, and so the rumors of the scary Men in Black from some cryptic Federal agency would be spread. If I can track that down, I will post it here.
Here's what I find really hilarious about the whole deal: Remember how Will Smith and Tommy Lee Jones would scour the tabloids for stories of alien sightings in the MIB movies, then go pay a visit to the witness? Well, according to the guy that wrote that article I am trying to find, that was precisely Barker's modus operandi: He'd find his victims in tabloid stories.
In 2008, a documentary about Barker called Shades of Gray was released, and is available on Amazon.