The Raven Priests do carry some significance, but it's not a link to Terrok or Skettis. I won't reveal why, exactly, the Raven Priests exist, but it's a sign that even the Crusade is losing a part of themselves up in Northrend. The most zealous crusaders on Azeroth, the ones who are most adamantly anti-Undeath, have lost their morals in the war. They have Death Knights in their ranks, and the Raven Priests represent a much darker side of their worship.
There is a very real force that's pushing these changes on the Crusade, but I'll let you find out for yourself (if you haven't already.)
Are the Knights of the Ebon Blade evil? The dialogue between Tirion and the Ebon Watcher (I won't say who he really is so as not to spoil it for anyone who hasn't been to Icecrown yet and reads these comments) seems to imply that they at least have a rather strong hatred of the holy light, which does send up some warning flags about them, as so far all light hating races have been evil.
They're not quite evil, and they don't really hate the Light. 'Morally grey' is a pretty good description of how they view things. They don't hate the Light, but they think Tirion's absolute faith is a shortcoming. To fight the Scourge, you need to get your hands dirty. While the Argent Crusade's actions are dictated by their faith, the Ebon Blade has absolutely no problem playing dirty and being rat bastards to advance their efforts against the Lich King.
Darion Mograine sees Tirion's devotion to the Light as something of a weak point, something the Lich King can (and will) exploit. There are things Tirion simply won't do. That's where the Ebon Blade comes in. They will do those things, because they realize that to win this war, they need to. You have to admit, how they operate has been pretty effective. Most of our victories in Icecrown were due to the Ebon Blade's involvement. Even Tirion's big moments, he had Darion Mograine and the Ebon Blade backing him up.
Sometimes you need to make sacrifices, and the Ebon Blade knows that. Tirion is a 'no man left behind' kind of guy, and when you're up against the Lich King, that only goes so far.
What's the deal with the 7th Legion in the Dragonblight? Are they just shock troops for the alliance or something more?
They're the Alliance's elite. Great warriors from all of the united Alliance races, and they've been on the front lines of all of the really major Alliance battles. As their flavor text mentions, they were at the Battle of Mount Hyjal, the recent Ahn'Qiraj war effort, and now in Northrend. We never saw them by name at Hyjal, but they were there in Silithus a few years ago, but not many people remember them. Wowwiki has pictures, if you're interested.
They're the Alliance's military badasses, but they're not royalty or politicians or anything like that. They're not as world famous as Alliance heroes like Jaina or Varian or Magni, but when they get called to the field they get shit done.
Ask a Lore Nerd is here to answer all of your questions about the lore and story of the Warcraft universe. From the religions of the universe to the 'evils' of Azeroth, everything is fair game. If you want more in-depth answers to some of your questions, you may find what you're looking for in Know Your Lore.