I am also of the opinion, if they have to hire a dark horse out-of-left-field guy, then the search has already failed, given the fact that ND is the only major program conducting an open coaching search this offseason (FSU already has their man on staff) and should have their pick at least of all the guys looking to move from smaller programs to that big job to become rich (i.e. Kelly, Peterson, Patterson, etc).
However, if they do end up hiring a dark horse, how about a black-as-night horse: Major Applewhite. The guy's an assistant head coach at Texas. He's not taking over for Mack Brown when he eventually steps down as Texas has the coach-in-waiting situation with Muschamp. He may have just coached Colt McCoy to a heisman. He had a solid career as a player at a premier program. He can open the pipeline to Texas in recruiting.
Don't get me wrong, I don't want him over any of the big names getting thrown around (Stoops, Kelly, Patterson, etc). But if the question is name a dark-horse, he's certainly that.