So I figure I should get off my ass and start writing for my dk, but im a bit stuck on her story.

tl;dr would it be possible for her to die in northrend during WotLK and be raised as a DK- but still become a knight of the ebon blade?

Yes, Arthas was still raising and training new death knights as we laid seige to Icecrown. You can tell because there’s a group of Death Knight adepts and Initiates just a bit northeast of the entrance of Icecrown citadel where you’re sent on a quest to kill an NPC. (The Black Knight was also raised as a death knight.)

The Ebon Blade is still accepting undead freed from the Lich King, as well; as evidenced by the fact that the three NPCs that you end up freeing from Arthas’ control (Vile, Lady Nightswood, and The Leaper) as you take the Shadow Vault end up joining the Ebon Blade.