In the world of Warj'Arune, all Blackguards must have an appropriate patron deity; after all, their power must come from someone. Inevitably, those deities that maintain a number of Blackguards in their service are Evil in alignment, just as a Blackguard must be Evil themselves.
Requirement: Prestige Class Trainer. In Warj'Arune, you must unlock the 1st level of this prestige class by visiting a temple of whatever deity you wish to become a Blackguard of, convert if you are not already a follower, and spending one or more nights in prayer and meditation. Whenever sufficient time has passed, the deity will begin granting you power (and access to this prestige class).
If a Knight Of The Last Stand or Divine Champion wishes to role-play a "fall-from-grace" story, and become a Blackguard, the player will need to seek out a DM and work it out with them. Likewise, a player with a Blackguard character who wishes to roleplay a "redemption" story must seek out a DM as well, and go through an arduous set of quests.