Under the current understanding of spirits, it would have to have been a blood spirit, yes. Of course, there are entities far older than the current understanding of magic and spirits out there. Yes, the Ghost Dance was a massive blood magic ritual. However, I do not think it was a blood spirit which taught Daniel Howling Coyote the Dance. My guess is that it was Thais, the half-horror 'son' of Aina Dupree, one of the Immortal Elves. Especially since it mentions in Worlds Without End that he tried to do something similar before the rise of mana had gotten high enough for the Dance to work. While we're on the subject, from an Earthdawn frame of mind there are two types of blood magic: Life and Death magic. The two types are differentiated primarily on the purpose of the spell/ritual, and the willingness of the participants. What the Azzies practice is primarily Death magic. Rituals like the Ghost Dance would be Life magic, with the participants willingly contributing their lives for the greater good. To put it in more mundane terms, both the German army in WWII and the Japanese Self-Defense Force today would be considered a 'military'. A diehard pacifist would decry the existence of either one. But on a practical level, there is a big difference between the two, especially when talking about size, scale, and the types of operations they're equipped and trained to do. Yes, this example is taking things to a bit of an extreme, but I only do it to illustrate the point.