User blog comment:The Joker~/How can I rekindle my interest in Skyrim?/@comment-4057437-20121129045941/@comment-1992639-20121130171349

If you played in a past version where the Blackreach had trouble respawning, it usually respawns after a very long time. Try not going there for a whole ingame year. I had the same issue. When I went back in 4E 403 there suddenly was new Crimson Nirnroot and NPC's respawned solidly from then on. Or perhaps installing Dawnguard forcefully reset it, that I don't know.