I'm in the theory camp that a BCV just isn't close enough to a human to be compatible as a host, given the "tainted by something hideous and unworldly". That phrasing of "unworldly" sounds like it might be a hint that they're linked to the Outside, and there's that WOJ that Harry telling Lash right before she sacrificed herself that he'd want answers about Maggie Sr. - Outsider - Black Court connection that it was not a type for "Black Council". If the Black Court are Outside-affiliated, and those of us who think Deirdre's talk of the Denarians "saving the world" are correct in interpreting that as save it from the Outsiders via scorched-earth tactics, a blampire might even be outright on the Fallen's enemies list. Even if that's not quite it, everything Nic said about hating the Red court (asthetically gross, and competing powers with growth potential are a threat to his own plans) goes for the Black as well.
Mavra probably will have had some sort of power growth to stay threatening (besides the soulfire and Winter powerups, there's also whatever the Word of Kemmler taught him necromancy could do to the BCV). But she can do that by having learned a MacGuffin of her own from Kemmler's book, and / or by coming back at the head of a scourge of a bunch of low-level mooks to distract Harry next time she shows up.
Although, like Harry himself, Mavra is quite capable of threatening adversaries of greater raw power by superior planning and sneakiness. With vampiric speed and her ability with veils, she could be incredibly dangerous just running around invisibly skewering people on a sword or breaking necks.
