Not Umbra Sumus, which is fine, but the series title, Dark Hunters. In WH40K lore, the Dark Hunters are a Space Marine chapter tasked with tracking down and destroying a Chaos Marine chapter known as the Punishers. Even by the grim standards of the setting, the Hunters are noted for being resolute and not much fun at parties.
The problem with this is that there is a quite well-known series by American urban fantasy superstar Sherrilyn Kenyon, also known as Dark-Hunter (I'm assuming the hyphen and singular title is what BL missed when seeing if the term was already copyrighted). It began in 2002 and now comprises 26 novels, accounting for the majority of Kenyon's 30 million+ sales. Although not often discussed on genre websites, it's one of the biggest series in the genre with sales far outstripping that of the likes of The Dresden Files.
Even the mighty Games Workshop knows better than to take on the legal forces of an author so popular she can make a logo out of her initials.
Umbra Sumus and the previous Dark Hunters WH40K material has been withdrawn and will be reissued after a title change, hopefully later this year. It's unclear at the moment if the BL will have to completely rename the Dark Hunters chapter in all of the lore as well.
Paul is also working on a new novel for Solaris, The Wolf in the Attic, which is now looking like an early 2016 release.