So under "Adding New Spells to the Repertoire, the core book says:
“All mages and elven spellswords are assumed to be members of the local mages’ guild, or apprenticed to a higher level NPC. When they gain a level of experience, they may return to their masters and be out of play for one game week per spell while they are adding new spells to their repertoire. Their masters will teach them spells equal to the number and level of spells the caster can use in a single day.”
So are nightblades NOT members a local guild and thus masterless, meaning that they can only learn new spells from scrolls or the spellbooks of other casters?
It’s up to you, but this is from the ACKS core rulebook -
“Elven nightblades cast spells as mages of one-half their level, using the same spell list and the same rules for learning and casting spells.”
It could be argued that your question falls under the “learning new spells” part of that statement.
However, since Nightblades are as much thief as mage, you could also have their thieves’ (or nightblade’s) guild hideout provide them with spell scrolls that the guild thinks are important for a nightblade to have access to when they level up… perhaps for a fee or service depending on the standing of the leveling nightblade in question.
Regardless, I would assume the nightblade would be a part of some guild or other, whether mage’s, thieves’, or nightblade’s.