So what about the Crusaders? In my version of Blackmoor, I have replaced them with Paladins. It should be noted that Paladins are mentioned in the First Fantasy Campaign. Paladins first appeared in published form for D&D with the Greyhawk supplement in 1975, which was available when the FFC was published (1977), but the references to Paladins in the FFC were more likely to Poul Anderson's Three Hearts and Three Lions.
The character of Zvenzen was probably roleplayed close enough to the Paladin concept to be called the first Paladin PC in Blackmoor even though the class did not exist when Greg Svenson created his character for the game.
But what can be gained from merging Paladins with the FFC idea of Crusaders? One thing is that when encountered these Crusaders are probably on what they see as a holy mission. In some cases they could be travelling to battle evil in the form of the Egg of Coot, the Afridhi or other monsters or demons. They could likely be in possession of Holy Swords as described in the FFC p 47. Or they could be questing for a Holy Sword, or other religious artifacts.
On page 49, one of the events that can occur by chance in the lands controlled by a player is that Crusaders are "passing through" the lands. This presents an opportunity for the PCs to recruit the unit of Crusaders (comparable to a mercenary unity as per the FFC), but they could also cause problems for the PCs if they have allowed morally grey activities to take place on their lands.
My impression about these travelling Crusaders is that they might likely often hail from the Great Kingdom/The Empire if Thonia and be encountered when travelling on their quests across the lands of the North.
In later version of Blackmoor, Zeitgeist Games released a free PDF (available on my website) detailing various organizations of Paladins for Blackmoor, but this PDF was fairly barebones and an adaptation of material from D&D 3E Unearthed Arcana. Still, it might have some merit to use this material to further develop different Crusader or Paladin Orders which is something that I am currently working on.