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 5th-level summons will be a 10HD creature, a 9th-level Blackguard's summons will be 14th-level, while a character with the full ten-level 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 or the Blackguard's Create Undead ability.
Note for legacy characters: If you have a character who had this feat before this change was made, you will need to speak to a DM about granting you the Release Binding feat. The feat number is 2654.