I've found the Blood Citadel and Vale of Infinite Horror. The first one is nice, the second one...well, is not nice.
There are some blood magic sites but they aren't very common. Some of them are very, very nice (Mountain of Power, Devil's Den) while others are just lame (that Villains site).
Blood magic was considered overpoweringly good in Dominions 2. I don't play that much with blood nations, so I don't know how strong it is in Dom 3.
Growth can nullify the poploss a bit, but it won't stop it.