Although I have not been able to find any, I assume that at one time publishers’ book lists were simple one-page price lists that salesmen used when selling to bookstores. Now many “lists” are thick color catalogs, website pages or PDF downloads.
The front, mid and back designations relate to the position of a book in a publisher’s catalog -- or state of mind. Being on the backlist is not necessarily an insult. Simon & Schuster’s backlisters include Pulitzer-Prize-winner David McCullough and Nobel-Prize-winner Ernest Hemingway.
A black list, on the other hand, is a list of things or people to avoid. Try not to be on one of those.
(From my 1001 Powerful Pieces of Author Advice