Injuries happen. It's morbid, but that's the way it is.
Along the same lines, if you were to sit down and really look at the backup running backs out there - and I mean, really look at every team and map it out - I think you'll find that LeGarrette Blount is among the most talented and perhaps the most experienced of those backups. Alfred Blue is certainly talented, but come on - Blount is far less risky for a lower free agent auction budget price.
Of course, Le'Veon Bell isn't injured and unless the game is out of hand, Blount isn't in line for double-digit carries. That doesn't make him any less valuable of a handcuff though, and it's not like Bell has been Mr. Ironman in his career. If you can afford the stash, Blount is worth it.