Blackguard's fiendish servant gets SR equaling class level +5 when the blackguard is at character level 19. The fiendish servant does have the fiendish template, so it has SR of HD +5. If the blackguard has 8 or fewer levels in blackguard, the servant gets more SR from being fiendish than it does from the blackguard. Even if he does finish, the horse will always have more SR.