I believe it was actually Albert K. Bender who came up with the Men In Black. Keel was only a popularizer.
"We advise those engaged in saucer work to please be very cautious."
Keel followed up on it and "documented" it as a widespread thing.
Barker essentially made it up and even probably was one of the "Men in Black" that scared the rather nutty Bender in the first place.
Barker's influence on society is also pretty big. All sorts of crazy ideas that have infested our pop culture come from him.