Each blood slave sacrificed during blood sacrifices spreads Dominion just like a temple check, which is performed once per turn at a temple of anyone who isn't Mictlan. So if you sacrifice 6 blood slaves every turn, it'll be as if there was a 6x (or 7x, if Vanheim/Abyssia, since the temple itself will perform a check) temple there.
The details of how a temple check spread Dominion can be found here:
http://www.shrapnelgames.com/cgi-bin...=000981#000011