Any follower is a candidate for the blades, wether housecarl or the followers from various bars, wether by coin or Irish Standdown ( a type of one on one fight). The same tricks which make any NPC into a regular follower from the console, will work here too, then you just turn them over to the Blades. I tryed recruiting a Vampire follower I had made of a generic vampire into the blades once in this way.