Darken's vocals aren't amazing (though they're quite good in earlier releases, particularly The Celtic Winter) but they don't in anyway detract from the utterly brilliant music (again, mostly referring to the earlier works). A typical shriek would almost certainly negatively affect Thousand Swords, I reckon.
I was expecting you to say some band that I really don't like, but there you go saying the band with my favorite vocals in this entire genre. How ironic.
I actually like both vocalists in Peste Noire. Famine's vocals are like some diseased plague-breathing corpse, which is perfect for their music, and Neige's vocals (such as on "Dueil Angoisseus") are just soaring and majestic. I'd say that overall, Neige is my favorite in the genre and I enjoy all the black metal bands he's appeared in to do vocals.