Summon Fiend has had several changes applied. They are more powerful than the pathetic dire rat from the OC, and a Blackguard character's summon grows more powerful as she takes more Blackguard levels. Lawful Evil and Neutral Evil Blackguards will gain a series of Excruciarchs (pain devil), while Chaotic Evil Blackguards will be able to summon several different battle-trained Succubi.
Advancement is similar to the summons of Demonologists, where the initial 4th-level summons will be a 10HD creature, a 9th-level Blackguard's summons will be 14th-level, while a character with the full progression of Blackguard with a total character level of 25 (inlcuding Level Adjustment) will be capable of summoning an advanced 17th-level creature. In addition, this creature will go into the Henchman slot rather than the summon slot, allowing it be used simultaneously with summon spells.
Note: with the change of blackguard from prestige class to base class part of the above is not valid anymore. All blackguard will get directly the 14th level henchman, and presumably the level 17 henchman at character level 25, if there are at least 10 levels of blackguard. This is probably going to be tweaked in the future.