WAG time!
I think most would agree that there as at least an implied connection between the Black Court of Vampires and the Outsiders.
They are often described as being tainted by something inhuman, unworldy and evil. Something that robs them of all choice.
Renfields are particularly interesting - their will is some brutally and completely overridden that the person they were before is essentially dead. They are empowered by that same fell strength that the Black Court runs off (though obviously a lot less than a BCV). They are described as being bound by a vile collar of dark energy (a reflection of the magic that enslaves them), their muscles are bulging with blackened veins, but the worst is the eyes - which appear as though they have been clawed out and their is a hideous cold darkness, vast and terrible instead. A profound Nothing. This is their description when Harry views one with his Sight. Curiously, neither the OG Merlin nor any Saint on record who tried to remove the spell was able. This suggests to me that once their minds/psyches have been so dominated so violently by the BCV, they are effectively dead. There is nothing left of the original person. Just a meat suit.
The Renfields, and vampire thralls, are an example of how the White Council's Laws can be perverted and used against them. The White Council cannot kill (with magic) mortal beings, even Renfields it seems, making them effective tools and meat shields. It was problems like this that required the creation of the position of the Blackstaff.
A few inconsistencies, or at least issues, exist though. In Blood Rites, Harry believes that the Black Court have been around since the dawn of human civilization and Ebenezar says that Dracula (son of Drakul) went to join the Black Court as a sort of teenage rebellion. However in WOJ we have a quote saying that Dracula, in an effort to impress his father (Drakul), wound up creating himself as the first Black Court Vampire - but it didn't work out so well. Finally, OG Merlin supposedly tried to remove or fix the Renfield curse/enthrallment. However, the OG Merlin would have existed about a thousand years before Dracula (who would have existed about 600 years before the Dresden Files).
So which is true? As the WOJ came after Blood Rites, most have taken that to mean Jim has retconned/rethought his original idea of the Black Court. Of course it could be a genuine mistake too, but perhaps the discrepancy was intentional (it would hardly be the first...). An easy solution to the Merlin issue is that he is a Time Traveler. But then, is Dracula? Did he go back to the dawn of humanity to become the first Black Court Vampire?
Also - Mavra is about 600 years old according to WOJ, and the name Mavra comes from a Russian opera about a love affair. Mavra is the pseudonym of a man (Vassili) pretending to be a servant. Is there a connection between Mavra and Russia? Well for one, Simon Pietrovich was Russian and the White Council's expert on Vampires. He is also the number one suspect for Cowl currently (as rated by these boards). It would fit - Jim loves those 80s references, the whole Russian vs America thing is very much a part of pop culture (and was heavily so in the 80s). Also, if Mavra is 600 years old she definitely existed during Dracula's reign. Which either makes her one of the oldest and strongest Black Court Vampires, but might also mean she was someone from history. Perhaps Dracula's sister/wife/mother etc.
I have also speculated in the past to the connection between Drakul and the Outsiders - which would make more sense if the Black Court were heavily connected to the Outsiders as well.