TL;DR: Siah Aimians were Honorspren that came over to the physical realm like Ishar is trying to do. That's why they are all blue and that's why Aimia was where all the ancient Fabrial's, spren turned into objects came from in the Silver Kingdoms. Axies can change his skin at will is the physical realm, this is a remnant of Spren like Syl changing her appearance in the physical realm. Axies is documenting every possible spren in the physical realm, building a family tree of sorts. Axies has been