Factual error: Every time the silver nitrate is shown on clothing, it is shown as silver. Silver nitrate in fact is black once exposed to light.
Factual error: Selene's guns are apparently capable of firing 30 rounds without reloading. Considering the size of the ammunition, this is physically impossible.
Chosen answer: In most of the shots, Lucien takes his coat off when he changes to wolf form, and the blade would be attatched to a mechanism in him coat. Also, the film gives us an example of the blade hurting a vampire: bloodloss. And that kind of weapon could do some serious damage, and as it probably wouldn't kill the vampires, it would almost definitely slow them down.