I think Laurell said it best on her MySpace when someone asked her what the book was about:
"BLOOD NOIR is about friendship and love, and telling the difference even when the boundaries are pretty blurred by conventional standards..."
"BLOOD NOIR taught me, and Anita, that we didn't know all that much. That, though Jason seemed like an open book, in fact, he'd kept most of his past hidden, at least from Anita and me. Did Nathaniel know more? They are like best guy friends, so maybe Nathaniel knew more, but neither of the men shared it with Anita. Of course, she did the girl sin, she didn't ask. To be honest, it never occurred to me to have her ask about Jason's past. He was just always there, our friend, our confidant, and sometimes lover for Anita. By the time I finished writing BLOOD NOIR I knew more about Jason's past than I do about any one's but Anita, Richard, and Jean-Claude..."
"So, what is the book about? It's about love, and friendship. It's about family and the damage they can do to you. It's about forgiveness. It's about how far you would go for one of your best friends, and what you'd do to keep them safe. Yeah, there's sex in the book, and it's damn good sex, thank you very much. But it's not what the book is about. There is violence in the book, and killing, and kidnapping, and torture. But that's not what the book is about either.
People get hung up on the sex and violence in my books, but just like real sex and violence, it seldom is what defines your life. Violence can define your death, but seldom your life. What's the book about? It's about love, and honor, and the price the innocent pay for the stupidity of others. It's about Jason letting Anita into more of his world, and taking me along for the ride. It's about a character that has been my imaginary friend for twelve years, finally being ready to tell me his story."
It is not as high action as some of her other books, but it is a very important one that teaches Anita about the importance of friendship (not that she didn't already know, but it showed her again.) I think it made it clearer to Anita what Jason meant to her and how far she was willing to go to keep the people she loved safe (kinda like with Chimera).