I currently tank HToC 25 with a Blood spec, and really like it
I've taken 3/5 Necrosis instead of 3/3 WotN after a small math. WotN allow me to avoid about max 2k7 hp from a hit when I'm full buffed. However, those 3 points may be used in an other talent, but I don't really know which (Ravenous Dead or Necrosis?)
I think Glyph of Diseases is a really usefull glyph in raid conditions. It allow me to do a cleaner, reactive rotation than a IT/PS refresh rotation, and to do one more HS. Sudden Doom scale with the use of a Pest rotation, and Morbidity scale with Sudden Doom (and a 15s CD DnD is always welcome in AoE Threat). All of those for a good single threat.
I confess I don't like lose hit pool (it's surely a fault), thinking hp pool remain the better survival point in HL PvE, even if I change for a avoidance stuff in some encounter or if I see "main stuff" of certain tank gemmed with Dodge/Sta, Def/Sta gems, or even Exp/Sta and Hit/sta.
I apologize for my bad english, I'm french