As to the two questions not answered by the above:
The idea was that if the character comes from blackguard (I understand some could not, such as evil paladins or the Disciple of Levistus from my previous thread), then they have a fiend already who they can murder to enter this class. Unearthed Arcana suggested it was a bearded devil that sponsored blackguardom, and I don't think anything stronger than an erinyes would take much an interest in mortals becoming blackguards. That being said, the disciple-in-training can easily just go out and find an imp, babau or tiefling and make friends with it, then off it.
And I do think it fits Bel, a pit fiend who clawed his way to the top. There's only one way to do that in Baator, killing the guy above you! (or getting him demoted, but Bel is a WARRIOR!!! THIS IS BAATOR!!!!)
I'll stat out a little blurb for the Standard of Bel.