That is why I thought that there ought to be some kind of reward, and incentive to invest more levels into the class. The same with most prestige classes really. (Except Palemaster and RDD, They have pretty sweet bonuses as it is.)
So, without further ado- This is the best suggestion we got out of the discussion so far.
Ork wrote:
Contagion — we could change this to something equivalent to 3.5's version of Aura of Despair. The blackguard radiates a malign aura that causes enemies within 10 feet of him to take -2 penalty on saving throws. This sounds a lot like a Barbarian's Terrifying Rage without the fear aura.
Inflict Serious/Critical Wounds — Pathfinder has an ability called Touch of Corruption which is practically the opposite of Lay on Hands. Blackguard level x Charisma bonus for damage. This wouldn't be as significant as the Paladin's version as the highest damage possible would be 20 x 15 = 300. We could cap it similar to harm. It would also have a touch attack die that needs to be rolled.
Epic Fiendish Servant — Turn the Blackguard summon into an abyssal/infernal shadow of himself. The summon has equal HD to 10+Blackguard Level. A full level 20 blackguard would have a duplicate of himself with a 30 HD on par with most druid companions.
Why this encourages going further into Blackguard? Touch of Corruption would only do 40 damage on a traditional 4 BG level dip (4 x 10 Cha) which would be hardly worth it. Epic Fiendish Servant would be a pretty awesome summon to tote around a duplicate of your evil self, and Aura of Despair would make fighting a Blackguard a frustrating affair.