[Taqwus]

Holy/Unholy sites do exist, just a bit rare.
A few sites do give blood income (Damned Merchant, for instance; there's also some non-blood magical academy site that does, too) or blood summoning bonuses, but they're far rarer than brigand lairs et al. But blood slaves are mostly captured, not received from sites. There's also a Demon Lord which gets free slaves, but not at a terribly quick rate.