How come the Dusk vampires didn't realize Evan was a werewolf?
That's a really good question! I think there are multiple explanations for this.
(Green) elves have a natural connection with beasts. They can sense when an animal is nearby even if they can't see it, and they can understand what an animal is thinking and feeling.
Werewolves are beasts, but lycanthropes are something else--something between man and beast. They are often described as "a beast wearing a man-suit". In a sense, lycanthropes are like intelligent beasts who can operate as people.
Theoretically, the elven Dusks should have been able to sense the beast in Evan. Why didn't they?
Let's talk about vampirism for a second. It changes elves by dulling their natural abilities and replacing them with new ones. This effect becomes stronger the longer they live with vampirism. For example, their naturally sharp vision dulls in daylight but becomes stronger at night. Their natural connection to animals becomes dulled, but their connection to the moon becomes stronger.
Knowing all this, here's some explanations for your question...
Explanation 1: All of the Dusks (except Lilian) have been vampires for a very long time. I think they all lost their connection to nature ages ago. Lilian still may have had this connection, but she was so sheltered and uneducated, I doubt she even knew what lycanthropy was, so she wasn't able to identify it in Evan.
Explanation 2: It's mentioned in "Ghoul Beneath the Guise" that the Dusks didn't prey on big, strong people. They specifically hunted smaller, weaker individuals for their own safety. Lycanthropy turns people into huge, muscle-bound tanks...in other words, not the type of people the Dusks would have close contact with, so they had very little--if any--experience with lycanthropes and kind of forgot they existed.
The Dusks didn't go out looking for Evan. They only imprisoned him because he came to them and practically threw himself onto their plates. The possibility of lycanthropy didn't cross their minds because as mentioned above, that disease had become out of sight, out of mind for them; probably for centuries.
Explanation 3: The Dusks already had their hands full when Evan showed up. They were trying to recruit Zeffer and prepare for his wedding with Lilian. Weddings are a huge deal to vampires because it's one of the few ways they can expand their clan size, and adding a new member to the clan is not something they take lightly.
This event was an especially big deal to Dario, considering how he felt about Lilian. Evan was the last thing on his mind; just another bloodbag to be thrown in the dungeon and forgotten about.
Explanation 4: Dimitri was a bad dungeon-tender. Dimitri was Dario's "nephew" and put in charge of keeping the Dusk's prisoners alive. As we saw in "Ghoul Beneath the Guise", the guy was a total sadist who enjoyed hurting and killing the prisoners, even though this got him in trouble with the clan. The more the prisoners suffered, the more Dimitri thrived.
Dimitri had a lot of responsibilities, and one of them was reporting sick, dead, or unsuitable prisoners to Dario. Dimitri should have noticed something was up with Evan's blood, and maybe he actually did...but in "Ghoul Beneath the Guise", we learn that Dimitri is terrified of Dario and tries to cover up problems whenever he can. Because whenever he reports a problem, even if it's not his fault, Dario gets angry and beats him.
It's possible that Dimitri did notice the tainted lycanthrope blood and either didn't report it, or just denied it to himself because psychologically he couldn't deal with it.
In conclusion, there was a perfect storm of factors that led to Evan's disease going undetected. And what's funny is that all of them were actually Dario's fault if you think about it...
Dario was the clan master, his job was to oversee the whole clan and its culture. He refused to educate Lilian, he chose who his clan preyed upon, he was too preoccupied with Lilian's wedding because of his own bad deeds against her, and he kept punishing Dimitri just for doing his job.
Evan's lycanthropy wasn't to blame for the Dusk's downfall. Ultimately Dario was to blame. The incompetent way he managed his clan finally came to a head and bit them all in the ass. Evan's disease was just a random spark that lit the fuse Dario left trailing behind them.
