When flanking an opponent without crit immunity, level 10 Blackguard does get an average +10.5 damage increase per hit from Sneak Attack, compared to having just went Fighter. Also some Saving Throw bonus, at times.
In what ways would you like Blackguard to be buffed and in comparison to what class(es)? (Paladins aren't a fair comparison -- They have ridiculously strict codes and rules and aren't lore-wise the counterparts of Blackguards and BG from Fighter enjoys Weapon Spec and in future, possibly other Fighter-only feats)