In short, this book is a 'secret history' comedy. It's set in nineteenth century England, and the main character is attempting to use forgery to cheat his way into a vast fortune. It doesn't go according to plan, so to speak, and he ends up being mistaken for a vampire and has to try to put everything together as things start to fall apart.
One great thing this book accomplishes well is what I would call a misdirected plot twist. It sets up the story in such a way that you think "A-Ha! This is what you're going to surprise us with," and then the book says "Oh, you thought this was the answer? Logical conclusion, yes, most people would think that. Except you're completely wrong." Those aren't direct quotes, but you get the idea.
But the best part about this book is that its the only book I've ever read that is so flawlessly filled with British humor. A lot of it is back and forth banter augmented with logical fallacies, along with absurdly convoluted structural arguments that it isn't even worth trying to piece together to forge sense out of. This is a Gothic tale about dark spooky nights, yet I wouldn't hesitate to shelve it with regular comedies.
Most historical fiction I read has an impossible time trying to grasp at my interest. I'm not quite sure what it is, but most of the historical works I've read are quite simply boring. I couldn't care less about the fictional childhood of Henry VI. Perhaps one of the main reasons for this is that a lot of the stuff I've read deals a lot with family ties and commitments, which are things I simply don't find interesting. They are certainly not things I would put at the forefront of any story. But A Night of Blacker Darkness isn't about family (kind of). Well, technically that's the main plot, but its sort of also not the main plot. It's a bit difficult to explain without spoiling anything, so I won't try to.
Tangents aside, if a Gothic vampire novel comprised entirely of British humor sounds like an interesting book to you, I'd highly recommend it. It isn't part of a series and prior knowledge of the history of that time period isn't necessary, but it certainly enhances the experience.