MiBs were a hoax perpetrated by ufologist prankster Gray Barker, he made them up. He originally called them "blackmen," but that title changed as did culture/society. It amazes me how they have grown in folklore.
There is a public library in WV that holds all of Barker's papers and correspondence. I have long wanted to spend a week or two going through the collection. I especially would like to see his correspondences with George Adamski and John Keel, he apparently played both of them quite extensively. Unfortunately, Mrs Duke's idea of a whoop-de-doo good time does not include sitting around in a rural library while I read the antics of a paranormal practical joker.