Good, but…
I first read The Black Prism, book one of The Lightbringer series, and found the political intrigue gripping but the characterization weak and repetitive. Good news: The Blinding Knife carries on—and deepens—the political action, and the characterization flaws have been improved. Particularly, Gavin and Kip no longer sound like the same person in their “self talk.” Gavin shines as his own very strong-willed character, fulfilling a potential hinted at in Book #1 but never delivered, while Kip shares inherited traits to which he gives his own little twists. I like that. I like seeing the growth of the characters (as individuals, please, not copies!).
I still found the magic sounding “plastic,” but the handling is smoother in this book, though the reader is often subjected to long (sometimes pages), dry lectures about how it works. It promotes a deep desire to skip ahead, as do the random conversations with seemingly little purpose. Filler, anyone?
The actual meat of the story itself is good. Really good. Engaging. Complicated. It is unnecessarily littered with the repetition of descriptions, phrases, favorite words (just for instance, how many times does the reader really need to be given the same description of Blackguard garb?). The swearing swiftly became gratuitous, the crassness and vulgarity often jarring. I’m not a fan of swearing or vulgarity and don’t think they’re necessary to a good story. “Profanity is the effort of a feeble mind to express itself forcefully.” That opinion aside, if they’re going to be used, use them constructively, and not with the appearance that a minimum requirement of “gross” has to be filled—quick, before the story ends. There are references (take the “Amazoi” women, for example) that show a sad lack of imagination when it is obvious that the writer could, with a moment or two of consideration, vastly improve on the well-known model. 
The repeatedly (or maybe misleadingly?) mistaken use of the word “bemused,” along with the plethora of incomplete sentences, nearly drove me batty. 
The sex scene? More gratuitousness. And awkward, too.
Such good material, yet the book is hindered by weak editing. I love a good, long fantasy, but this one felt long.
The cover is gorgeous. I do like the blend of gunpowder and magic, and how the existence of one doesn’t detract from the other. I like the slick way the Color Prince is developed as a real person with real beliefs, and not just a cardboard cutout antagonist. The color wights are wonderfully, imaginatively, frighteningly depicted. The story of the real Gavin is disappointing. Lots of building up goes into his non-part; I expected more. I liked learning more about Ironfist’s background, and his inner workings. He’s one of the better characters in the tale. When the pace of the story is let free to go, it really moves!
[This book was received free through Goodreads First Reads.]