I'll throw out a few ideas so that people will have something to respond to.
First of all, the RAW prereqs for the blackguard are as follows:
Alignment: any evil.
BAB: +6
Skills: hide 5 ranks, knowledge (religion) 2 ranks
Feats: cleave, improved sunder, power attack
Special: The character must have made peaceful contact with an evil outsider who was summoned by him or someone else.
I think they mean "conjured" or "called" in that last line. Otherwise the requirement is satisfied by someone casting Summon Monster I to get a fiendish dire rat, but is not satisfied if someone gates in Asmodeus (even if Asmodeus is peaceful).
I think the alignment and special requirements should remain the same. The knowledge (religion) requirement also makes sense. I'm a little less certain about the BAB requirement. It would add a delay for rogues, bards and clerics, but maybe that's OK.
To think about what other prerequisites would be appropriate, it might be helpful to see what blackguards do:
They have an aura of evil. The aura class feature (possessed by paladins and clerics) might work as a prerequisite.
They detect good. This is the mirror image of the paladin ability. Clerics can also use the spell.
They command undead. Again, levels of cleric would be good, or several levels of paladin.
They add their charisma bonus to their saves, just like paladins, and they have the "smite good ability." Again, the anti-paladin theme.
They use poison. No base class has this ability, but the assassin does. Borrowing from the prerequisites of the assassin's might work (especially appropriate might be the "kill someone for no other reason than to join the assassins" prerequisite).
They have a fiendish servant. This is the mirror image of the paladin's mount, but is analagous to a wizard's familiar or a ranger's animal companion.
They cast spells. Some kind of spellcasting (perhaps divine spellcasting) might be an appropriate prereq.
They have sneak attack damage. Levels of rogue, maybe?
It strikes me that there are a *lot* of (anti) parallels with paladins. Which makes it especially strange that a fallen paladin can't become a blackguard without a lot of peculiar feats and skills.
So, I'd suggest the following:
alignment: any evil
The character must have made peaceful contact with an evil outsider who was called by him or someone else.
The character must also kill someone for no other reason than to become a blackguard. (Their victim is often specified by the outsider, but sometimes the sacrifice of an innocent is part of the calling ceremony.)
skills: knowledge religion (2 ranks)
The character must have three of the following class features (although some may have been lost due to alignment violations):
moderate evil aura (see the detect evil spell description)
ability to smite (e.g. a paladin's smite evil or the destruction domain's granted power; the improved sunder feat also counts)
a familiar, paladin's mount or animal companion (basically, a creature who has the share spells ability)
spellcasting ability
ability to turn or rebuke undead
add their charisma bonus to their saving throws
sneak attack +2d6
etc.
There should be some requirement that makes it so a character has to be at least 5th or 6th level to qualify. Otherwise an evil cleric1 with the destruction domain qualifies.
Dunno what an evil bard can do, except take a level or two of cleric.
