Do black magic have a way to detect incest or counteract genetic problems? Since "free love" is a big thing among them and harems seens to form easily, there probably big clusters of genetic bottlenecks.
I think it depends on the number of black magic users there are, and if they only conceive with other magic user, and if they can have offspring with non-humans, and if those non-humans participate...
Seems to me with enough individuals and enough debauchery, you end up with hodgepodge enough assortment that inbreeding isn't as big of a risk, because it needs to be long term to get particularly bad. However, they seem to have distinct 'family lines' mentioned here, so it is likely that they do engage in some amount of monitoring? Unless they are just full Crusader Kings.